rt2x00: Don't alter rt2x00dev->default_ant

rt2x00dev->default_ant should be initialized once by the driver,
and should not be changed afterwards. Because rt2x00lib_config_antenna()
was using a reference to the struct antenna_setup it actually had the oppurtunity
to change the default antenna setting and it actually did that during the validation.

Instead of passing a pointer to antenna_setup the entire structure should be copied.

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