mwifiex: fix uninitialized variable access in pcie_remove

Checking the firmware status from PCIe register only works
if the register is available, otherwise we end up with
random behavior:

drivers/net/wireless/marvell/mwifiex/pcie.c: In function 'mwifiex_pcie_remove':
drivers/net/wireless/marvell/mwifiex/pcie.c:585:5: error: 'fw_status' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This makes sure we treat the absence of the register as a failure.

Fixes: 045f0c1b ("mwifiex: get rid of global user_rmmod flag")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>