net: socionext: remove set but not used variable 'pkts'

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

drivers/net/ethernet/socionext/netsec.c: In function 'netsec_clean_tx_dring':
drivers/net/ethernet/socionext/netsec.c:637:15: warning:
 variable 'pkts' set but not used [-Wunused-but-set-variable]

It is not used since commit ba2b2321 ("net: netsec: add XDP support")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>