rt2x00: Fix EIFS timing value

Olivier reported a difference between the EIFS
values used in the legacy driver and the one in
the rt2x00 drivers.

In rt2x00 the value was
	( SIFS + (8 * (IEEE80211_HEADER + ACK_SIZE)) )
which comes down to 314us while the legacy driver uses the value 364us

This was caused because EIFS is: SIFS + DIFS + AckTime
This patch will fix this by adding the DIFS by the above value,
and creating a SHORT_EIFS define which uses the SHORT_DIFS.

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