rt2x00: Check for specific changed flags when updating the erp config

Previously rt2x00 was always updating all erp related config variables
even though mac80211 might only have changed one. Hence, pass the
changed flags to the config_erp driver callback so that the driver
can limit the changes to the correct values.

This fixes an issue in AP mode where the beacon interval is not
initialized (and thus zero) but still sent to the hardware causing an
interrupt storm on rt2800pci hanging the system.

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