mlxsw: core: Change emad trap group settings

Currently, the emad trap init was done in the core. In the future we will
want to add some changes to the traps groups, according to device type.
This commit create a driver function to create the trap group for the
emad, so later it can be changed by devices. It also changes the emad
registration to use the new generic functions.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>