rt2x00: Use GFP_KERNEL for rx buffer allocation on USB devices

Since the RX path on USB devices is handled in process context we can
use GFP_KERNEL for RX buffer allocation. This should reduce the
likelihood of allocation failures.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Tested-By: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>