net: dsa: mt7530: fix port mirroring for MT7988 SoC switch

The "MT7988A Wi-Fi 7 Generation Router Platform: Datasheet (Open Version)
v0.1" document shows bits 16 to 18 as the MIRROR_PORT field of the CPU
forward control register. Currently, the MT7530 DSA subdriver configures
bits 0 to 2 of the CPU forward control register which breaks the port
mirroring feature for the MT7988 SoC switch.

Fix this by using the MT7531_MIRROR_PORT_GET() and MT7531_MIRROR_PORT_SET()
macros which utilise the correct bits.

Fixes: 110c18bf

 ("net: dsa: mt7530: introduce driver for MT7988 built-in switch")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>