net: dsa: bcm_sf2: Utilize BRCM_PSEUDO_PHY_ADDR

Utilize the newly introduced BRCM_PSEUDO_PHY_ADDR constant from
brcmphy.h instead of open-coding the Broadcom Ethernet switches
pseudo-PHY address (30).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>