mlxsw: spectrum: Push code getting port speed into a helper

Currently PTP code queries directly PTYS register for port speed from
work scheduled upon PUDE event. Since the speed needs to be used for
SPAN buffer size computation as well, push the code into a separate
helper.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>