rt2x00: Don't initialize beacon interval to 0 on rt2800 devices

Activating the TBTT interrupt when a beacon interval of 0 is configured
results in an interrupt storm causing the machine to hang. Hence,
initialize the beacon interval to a reasonable default of 100TUs.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>