sun/niu: fix wrong RXMAC_BC_FRM_CNT_COUNT count

[ Upstream commit 155b23e6e53475ca3b8c2a946299b4d4dd6a5a1e ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>