rt2x00: Calculate tx status fifo size instead of hardcoding it

Instead of hardcoding the tx status fifo size as 512 calculate it based
on the number of tx queues and the number of entries per queue. Also
round the size up to a power of 2 as kfifo would otherwise round it
down.

On rt2800pci this will increase the kfifo size from 512 bytes to 1024
bytes which is then able to hold the tx status for all entries in all
tx queues.

Furthermore, if the number of tx queues or tx entries changes in the
future (use of the MGMT queue for example) the kfifo size doesn't need
to be updated.

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>