mwifiex: fix compile warning of unused variable

We got a compile warning shows below:

drivers/net/wireless/marvell/mwifiex/sdio.c: In function
'mwifiex_sdio_remove':
drivers/net/wireless/marvell/mwifiex/sdio.c:377:6: warning: variable
'ret' set but not used [-Wunused-but-set-variable]

Per the code, it didn't check if mwifiex_sdio_read_fw_status
finish successfully. We should at least check the return of
mwifiex_sdio_read_fw_status, otherwise the following check of
firmware_stat and adapter->mfg_mode is pointless as the device
is probably dead.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>