rt2x00: Kill all tasklets during device removal

During device removal all pending work and tasklets must
be guaranteed to be halted. So far only the txstatus_tasklet
was killed.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>