atl1: remove set but not used variable 'advertising'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/atheros/atlx/atl1.c: In function 'atl1_set_link_ksettings':
drivers/net/ethernet/atheros/atlx/atl1.c:3280:6: warning:
 variable 'advertising' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>