net: dsa: mv88e6xxx: fix BMSR error to be consistent with others

Other errors accessing the registers in mv88e6352_serdes_pcs_get_state()
print "PHY " before the register name, except for the BMSR. Make this
consistent with the other error messages.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>