rt2x00: Decrease alignment headroom

We only need 4 bytes of headroom for alignment
purposes in the RX frame. It was previously higher
for optimization purposes which are no longer
possible due to DMA mappings.

v2: Fix patch error

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>