mac80211: don't return -EINVAL upon iwconfig wlan0 rts auto

This patch avoids returning -EINVAL upon iwconfig wlan0 rts auto. If
rts->fixed is 0, then we should choose a default value instead of failing.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>