rt2x00: rt2800: don't overwrite SIFS values on erp changes

The SIFS value is a constant and doesn't need to be updated on erp changes.
Furthermore the code used 10us for both, the OFDM SIFS and CCK SIFS time
which broke CTS protected 11g connections (see patch "rt2x00: rt2800: update
initial SIFS values" for details).

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