sun/niu: fix wrong RXMAC_BC_FRM_CNT_COUNT count

RXMAC_BC_FRM_CNT_COUNT added to mp->rx_bcasts twice in a row
in niu_xmac_interrupt(). Remove the second addition.

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>