wifi: mwifiex: handle possible sscanf() errors

Return -EINVAL on possible 'sscanf()' errors in
'mwifiex_regrdwr_write()' and 'mwifiex_rdeeprom_write()'.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230802160726.85545-3-dmantipov@yandex.ru