rt2x00: Add dev_flags to rx descriptor

The rxdone_entry_desc structure contains 3 fields
which are always 1 or 0. We can safe 8 bytes by
replacing them with a single dev_flags fields which
contain the flags for those settings.

Additionally we can remove the OFDM flag since it
is no longer used since the introduction of the
SIGNAL_PLCP flag.

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