rt2x00: Remove ENTRY_TXD_OFDM_RATE

The flag ENTRY_TXD_OFDM_RATE isn't flexible enough
to indicate which rate modulation should be used for
a frame. This will become a problem when 11n support
is added.

Remove the flag and replace it with an enum value which
can better indicate the exact rate modulation.

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