mlxsw: reg: Make flow counter set type enum to be shared

This is done as a preparation before introducing support for neighbor
counters. The flow counter's type enum is used by many registers, yet,
until now it was used only by mgpc and thus it was private. This patch
updates the namespace for more generic usage.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>