net: dsa: mv88e6xxx: remove duplicated register definition

There are two identical MV88E6XXX_PTP_GC_ETYPE definitions in ptp.h,
and MV88E6XXX_PTP_ETHERTYPE in hwtstamp.h which all refer to the
exact same register. As the code that accesses this register is in
hwtstamp.c, use the hwtstamp.h definition, and remove the
unnecessary duplicated definition in ptp.h

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>