rt2x00: rt2x00dev: use rt2x00dev->bcn->limit

The beacon data queue is initialized already,
so 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->bcn->limit value allows us to get rid
of a superfluous pointer dereference.

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