mwl8k: fix band for supported channels

mwl8k: fix band for supported channels

The band field for the supported channels were left unpopulated, making
them default to 0 == IEEE80211_BAND_2GHZ, even for the 5GHz channels.

This resulted in null pointer accesses if anything tries to access
wiphy->bands[channel->band] of a 5GHz channel on 5GHz only cards, since
wiphy->bands[2GHZ] is NULL for them (e.g. cfg80211_chandef_usable does).

Example kernel OOPS:

[  665.669993] Unable to handle kernel NULL pointer dereference at virtual address 00000016
[  665.678194] pgd = c6d58000
[  665.680941] [00000016] *pgd=06f8a831, *pte=00000000, *ppte=00000000
[  665.687303] Internal error: Oops: 17 [#1]
(...)
[  666.116373] Backtrace:
[  666.118866] [<bf0368dc>] (cfg80211_chandef_usable+0x0/0x1bc [cfg80211]) from [<bf025e64>] (nl80211_leave_mesh+0x244/0x264 [cfg80211])
[  666.130919]  r7:c6d12100 r6:0000143c r5:c0611c48 r4:c0611b98
[  666.136668] [<bf025d84>] (nl80211_leave_mesh+0x164/0x264 [cfg80211]) from [<bf02634c>] (nl80211_remain_on_channel+0x2a0/0x358 [cfg80211])
[  666.149074]  r7:c6d12000 r6:c6d12000 r5:c6f4f368 r4:00000003
[  666.154814] [<bf0262ec>] (nl80211_remain_on_channel+0x240/0x358 [cfg80211]) from [<bf02ddb0>] (nl80211_set_wiphy+0x264/0x560 [cfg80211])
[  666.167150] [<bf02db4c>] (nl80211_set_wiphy+0x0/0x560 [cfg80211]) from [<c01f94e0>] (genl_rcv_msg+0x1b8/0x1f8)
[  666.177205] [<c01f9328>] (genl_rcv_msg+0x0/0x1f8) from [<c01f89a0>] (netlink_rcv_skb+0x58/0xb4)
[  666.185949] [<c01f8948>] (netlink_rcv_skb+0x0/0xb4) from [<c01f931c>] (genl_rcv+0x20/0x2c)
[  666.194251]  r6:c6f70780 r5:0000002c r4:c6f70780 r3:00000001
[  666.199973] [<c01f92fc>] (genl_rcv+0x0/0x2c) from [<c01f8418>] (netlink_unicast+0x154/0x1f4)
[  666.208449]  r4:c785ea00 r3:c01f92fc
[  666.212057] [<c01f82c4>] (netlink_unicast+0x0/0x1f4) from [<c01f8790>] (netlink_sendmsg+0x230/0x2b0)
[  666.221240] [<c01f8560>] (netlink_sendmsg+0x0/0x2b0) from [<c01cccf8>] (sock_sendmsg+0x90/0xa4)
[  666.229986] [<c01ccc68>] (sock_sendmsg+0x0/0xa4) from [<c01cdcb0>] (__sys_sendmsg+0x290/0x298)
[  666.238637]  r9:00000000 r8:c0611ec8 r6:0000002c r5:c0610000 r4:c0611f64
[  666.245411] [<c01cda20>] (__sys_sendmsg+0x0/0x298) from [<c01cf52c>] (sys_sendmsg+0x44/0x6c)
[  666.253897] [<c01cf4e8>] (sys_sendmsg+0x0/0x6c) from [<c00090a0>] (ret_fast_syscall+0x0/0x2c)
[  666.262460]  r6:00000000 r5:beeff96c r4:00000005

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>