rt2x00: Disable txstatus tasklet by default

Enable the txstatus tasklet when interrupts are enabled and disable it
together with the interrupts. Also make the txstatus tasklet useful even
without the tx status FIFO and make use of the generic rt2x00 tasklet
initialization macro.

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>