rt2x00: Introduce tasklets for interrupt handling

No functional changes, just preparation for moving interrupt handling to
tasklets. The tasklets are disabled by default. Drivers making use of
them need to enable the tasklets when the device state is set to IRQ_ON.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-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>