mwifiex_usb_probe() defines and sets bcd_usb but does not use it,
Compiling with W=1 gives the following warning, fix it.
mwifiex/usb.c: In function ‘mwifiex_usb_probe’:
mwifiex/usb.c:383:41: warning: variable ‘bcd_usb’ set but not used [-Wunused-but-set-variable]

The unused variable seems to be present since 4daffe35 which introduced
mwifiex_usb_probe().

Fixes: 4daffe35 ("mwifiex: add support for Marvell USB8797 chipset")
Cc: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kirtika Ruchandani <kirtika@google.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>