rt2x00: Add Signal type flag

Instead of using the PLCP flag to indicate if the
signal value is plcp or the bitrate we should add
a new flag to mark the bitrate type explicitely.
This is usefull when new types are added later for
rt2800.

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