rt2x00: Don't enable short preamble for 1MBs

The timing settings for 1MBs should exclude
the short preamble bit since that only applies
to 2MBs, 5.5MBs and 11MBs.

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