rt2x00: Only set the TBCN flag when the interface is configured to send beacons.

These flags used to be fixed to one in rt2500pci_config_type, which
caused the beacon timer interrupt to fire. This would lead to
rt2x00lib_beacondone adding work which called
rt2x00lib_beacondone_scheduled which called ieee80211_beacon_get which
printed an error about not having any beacon data.

With this patch, these interrupts are only generated when the interface
is configured to send beacons.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>