net: mventa: remove unused variable 'dummy' in mvneta_mib_counters_clear()

net: mventa: remove unused variable 'dummy' in mvneta_mib_counters_clear()

Fixes the following W=1 kernel build warning(s):

drivers/net/ethernet/marvell/mvneta.c:754:6: warning:
 variable 'dummy' set but not used [-Wunused-but-set-variable]
  754 |  u32 dummy;
      |      ^~~~~

This variable is not used in function mvneta_mib_counters_clear(), so
remove it to avoid build warning.

Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>