net: dsa: bcm_sf2: add function finding RGMII register

Simple macro like REG_RGMII_CNTRL_P() is insufficient as:
1. It doesn't validate port argument
2. It doesn't support chipsets with non-lineral RGMII regs layout

Missing port validation could result in getting register offset from out
of array. Random memory -> random offset -> random reads/writes. It
affected e.g. BCM4908 for REG_RGMII_CNTRL_P(7).

Fixes: a78e86ed ("net: dsa: bcm_sf2: Prepare for different register layouts")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>