mwifiex: correct event header length

mwifiex: correct event header length

While decoding received event packet from firmware, 4 bytes
of interface header are already removed unconditionally.
So for handling event only 4 more bytes needs to be pulled.
This is achieved by changing event header length to 4.

Almost all the events, except BA stream related and AMSDU
aggregation control events, do not have the payload in their
event skb. Such events handling depends only on the event ID.
This event ID is the first four bytes of the event skb, which
is copied to a separate variable before pulling the skb header.
Hence event handling worked only for those events that didn't
have payload in event skb.

This patch fixes the broken event path of the events with
payload in their event skb without harming existing working
event path for the events without payload.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>