generic: b53: clear SM_SW_FWD_MODE unconditionally when enabling VLAN

Setting this bit stops BCM53125 (bgmac actually) from receiving any
packets. This bit is cleared conditionally in b53_switch_reset and it
seems the same is done in bcmrobo.c which never sets that bit again.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 35723