rt2x00: Copy the MAC address to the WCID entry properly.

Use the specific mac field of the wcid_entry structure to copy the MAC
address to, instead of just overwriting the structure.
Previous code resulted in the same, but this form is cleaner.

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>