rt2x00: rt2800usb: use rt2x00dev->rx->limit

The RX data queue is initialized already when
the rt2800_usb_enable_radio() function is called.

Fetch the number of the queue entries from that
instead of using the entry_num field of the data
queue descriptor.

The two values are the same, and the use of the
rt2x00dev->rx->limit value allows us to get rid
of a superfluous pointer dereference.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>