wifi: mwifiex: simplify PCIE write operations

Since 'mwifiex_write_reg()' just issues void 'iowrite32()',
convert the former to 'void' and simplify all related users
(with the only exception of 'read_poll_timeout()' which
explicitly requires a non-void function argument).

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230919132804.73340-1-dmantipov@yandex.ru