rt2x00: rt2x00dev: defer operational mode detection

Only do it after the queues are allocated. This
will allow to use the 'rt2x00dev->bcn->limit'
instead of 'rt2x00dev->ops->bcn->entry_num'.

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>