rt2x00: fix the definition of rt2x00crypto_rx_insert_iv

Remove the redundant l2pad parameter from the definition of
rt2x00crypto_rx_insert_iv which is used when only CONFIG_RT2500PCI but
none of the other rt2x00 family drivers is configured.

Signed-off-by: Andrew Price <andy@andrewprice.me.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>