rt2800: zero MAC_SYS_CTRL bits during BBP and MAC reset

Zero all other bits than RESET_CSR and RESET_BBP when want to do the
reset, that the vendor driver behaviour.

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