rt2x00: Interface sequence lock doesn't have to disable interrupts.

This lock is only used in the TX path and thus in process context. Therefore
we can use a much lighter spinlock variant.

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