net: dsa: mt7530: add the missing RxUnicast MIB counter

Add the missing RxUnicast counter.

Fixes: b8f126a8

 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>