rt2x00: remove intf->mac field.

The mac field of the rt2x00_intf structure is written to once and used
twice. In both these uses the mac address is available via other means.

Remove this field as it does not appear to be necessary.

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