rt2x00: allow txstatus_fifo w/o txstatus_tasklet

When DRIVER_REQUIRE_TXSTATUS_FIFO is set, intialize the
txstatus_fifo, but initialize rt2x00dev->txstatus_tasklet
only when both DRIVER_REQUIRE_TXSTATUS_FIFO and
rt2x00dev->ops->lib->txstatus_tasklet are set.

This allows the txstatus_fifo to be used by rt2800usb which
does not use txstatus_tasklet.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>