generic: mt7530: set CPU port to fallback mode

generic: mt7530: set CPU port to fallback mode

Currently, setting a bridge's self PVID to other value and deleting
the default VID 1 renders untagged ports of that VLAN unable to talk to
the CPU port:

	bridge vlan add dev br0 vid 2 pvid untagged self
	bridge vlan del dev br0 vid 1 self
	bridge vlan add dev sw0p0 vid 2 pvid untagged
	bridge vlan del dev sw0p0 vid 1
	# br0 cannot send untagged frames out of sw0p0 anymore

That is because the CPU port is set to security mode and its PVID is
still 1, and untagged frames are dropped due to VLAN member violation.

Set the CPU port to fallback mode so untagged frames can pass through.

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