net: aquantia: Make local functions static

Fixes the following sparse warnings:

drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c:224:5: warning:
 symbol 'aq_ethtool_get_coalesce' was not declared. Should it be static?
drivers/net/ethernet/aquantia/atlantic/aq_ethtool.c:245:5: warning:
 symbol 'aq_ethtool_set_coalesce' was not declared. Should it be static?

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