rt2x00: Introduce RXDONE_SIGNAL_MASK mask

Improve error message reporting when a frame was received
with unknown rate. Instead of using the boolean check if
the frame is supposed to be a PLCP value or not, we should
add a new mask (RXDONE_SIGNAL_MASK) which returns the type
identification for a signal value (i.e. PLCP). At the moment
we only have 2 different types, but more will arrive when
support for 11n is added.

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