wil6210: fix for 64-bit integer division

wil6210: fix for 64-bit integer division

On some platforms, cycles_t is 64-bit, and gcc generates call to
__udivdi3 for straight division of cycles_t/cycles_t. This leads
to compilation failure, as this function is not exist in the kernel
runtime. do_div() to rescue

Original report:

tree:   git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-next.git master
head:   2e91606f5e1ec7329557dfc0e298c4c021acbb80
commit: 7c0acf868d2e470c9d6a40091acf8d6444c01b57 [81/103] wil6210: Tx performance monitoring
config: i386-randconfig-ha3-0620 (attached as .config)

All error/warnings:

   drivers/built-in.o: In function `wil_vring_debugfs_show':
>> debugfs.c:(.text+0x39b9be): undefined reference to `__udivdi3'

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>