ionic: Remove set but not used variable 'sg_desc'

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

drivers/net/ethernet/pensando/ionic/ionic_txrx.c: In function 'ionic_rx_empty':
drivers/net/ethernet/pensando/ionic/ionic_txrx.c:405:28: warning:
 variable 'sg_desc' set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>