net: aquantia: Make function hw_atl_utils_mpi_set_speed() static

Fixes the following sparse warning:

drivers/net/ethernet/aquantia/atlantic/hw_atl/hw_atl_utils.c:508:5: warning:
 symbol 'hw_atl_utils_mpi_set_speed' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>