rtl8180: adapt for deprecated IEEE80211_CONF_SHORT_SLOT_TIME flag

This updates rtl8180 handling for short slot after "mac80211: fix short
slot handling". Only rtl8180_rtl8225 actually had code for handling
short slot times, so the other RF devices are untouched by this change.

Signed-off-by: John W. Linville <linville@tuxdriver.com>