ptp: add stub function for ptp_get_msgtype()

Added the missing stub function for ptp_get_msgtype().

Fixes: 036c508b ("ptp: Add generic ptp message type function")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>