wifi: libertas: prefer kstrtoX() for simple integer conversions

Prefer 'kstrtoX()' family of functions over 'sscanf()' to convert
strings to integers and always check results of the conversions.

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

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230725060531.72968-6-dmantipov@yandex.ru