wifi: libertas: handle possible spu_write_u16() errors

Check and handle (well, report at least, as it's done through the rest
of the module) possible 'spu_write_u16()' errors in 'if_spi_e2h()'.

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/20230725060531.72968-5-dmantipov@yandex.ru