rt2x00: Optimize configuration handling

Implement latest changed from mac80211 configuration
handling to optmize configuration handling in rt2x00.

 * Remove set_retry_limit callback function, handled
   through config()
 * Move config_antenna to its own callback function,
   it isn't handled by mac80211 anymore
 * Use IEEE80211_CONF_CHANGED_* flags and remove manual
   checks
 * Removed deprecated short slot setting through config()
   and put it in config_erp() through which mac80211 now
   configures it
 * Remove config_phymode() and move contents to config_erp()
   since it only managed the basic rates which is now
   determined by mac80211 through config_erp().

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