mlxsw: core: Introduce support for asynchronous EMAD register access

So far it was possible to have one EMAD register access at a time,
locked by mutex. This patch extends this interface to allow multiple
EMAD register accesses to be in fly at once. That allows faster
processing on firmware side avoiding unused time in between EMADs.
Measured speedup is ~30% for shared occupancy snapshot operation.

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