mlxsw: spectrum: mr: Make the function mlxsw_sp_mr_dev_vif_lookup static

The function is only used internally in spectrum_mr.c and is not declared
in the header file, thus make it static.

Cleans up sparse warning:
symbol 'mlxsw_sp_mr_dev_vif_lookup' was not declared. Should it be static?

Fixes: c011ec1b

 ("mlxsw: spectrum: Add the multicast routing offloading logic")
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>