rt2x00: Use GFP_KERNEL for rx buffer allocation on USB devices

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 <[email protected]>
Acked-by: Gertjan van Wingerde <[email protected]>
Tested-By: Marc Dietrich <[email protected]>
Signed-off-by: John W. Linville <[email protected]>