wifi: rt2x00: simplify rt2x00crypto_rx_insert_iv()

In 'rt2x00crypto_rx_insert_iv()', added alignment can't exceed 3
bytes and ICV size is either 4 or 8 bytes, so skb space adjustment
may be simplified. Compile tested only.

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://msgid.link/20231019070750.17911-1-dmantipov@yandex.ru