liquidio: remove set but not used variable 'is25G'

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

drivers/net/ethernet/cavium/liquidio/lio_ethtool.c: In function 'lio_set_link_ksettings':
drivers/net/ethernet/cavium/liquidio/lio_ethtool.c:392:6: warning:
 variable 'is25G' set but not used [-Wunused-but-set-variable]

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