mlxsw: spectrum: Rename the firmware file

Change the firmware file name to be in "mellanox" directory.

This commit is a followup to the linux-firmware commit a4c72696f5f4
("Mellanox: Add firmware for mlxsw_spectrum")

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