rt2x00: Force full register config after start()

rt2x00 will only perform configuration changes from
mac80211 when the configuration option has changed.
This means it keeps track of the current active configuration
and will check these values when the config() callback function
is used.

However this causes breakage when the interface has been
brought down and up again, since all stored active values
aren't reset while the registers might have.
This is for example the case with rt61pci antenna registers which
will jump to invalid values when the interface has been started.

To make sure a full configuration takes place after the start()
callback function, a new flag is added which will be checked
during config() and skips the "what's changed" phase.

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