rt2x00: Use the available helper functions to initialize the WCID table

Use rt2800_config_wcid and rt2800_delete_wcid_attr to initialize the
WCID table instead of writing to the registers directly.

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