generic: mt7530: fix null pointer dereferencing in port5 setup

generic: mt7530: fix null pointer dereferencing in port5 setup

The 2nd gmac of mediatek soc ethernet may not be connected to a PHY
and a phy-handle isn't always available.
Unfortunately, mt7530 dsa driver assumes that the 2nd gmac is always
connected to switch port 5 and setup mt7530 according to phy address
of 2nd gmac node, causing null pointer dereferencing when phy-handle
isn't defined in dts.
This commit fix this setup code by checking return value of
of_parse_phandle before using it.

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>