rt2x00: Fix BUG_ON() with antenna handling

With the new configuration handling, and more specifically
splitting the configuration of the antenna from the normal
configuration steps allowed a BUG_ON() to be triggered
in the driver because the SW_DIVERSITY was send to the
driver. This fixes that by catching the value early in
rt2x00config.c and replacing it with a sensible value.

This also fixes a problem where the antenna is not being
initialized at all when the radio is enabled. Since it
no longer is part of the mac80211 configuration the
only place where rt2x00 configured it was the SW diversity
handler. Obviously this is broken for all non-diversity
hardware and breaks SW diversity due to a broken initialization.

When the radio is enabled the antenna will be configured
once as soon as the config() callback function is called.

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