net/mlx5e: Remove unused variable

A cleanup removed the only user of this variable

mlx5/core/en_ethtool.c: In function 'mlx5e_set_channels':
mlx5/core/en_ethtool.c:546:6: error: unused variable 'ncv' [-Werror=unused-variable]

Let's remove the declaration as well.

Fixes: 639e9e94 ("net/mlx5e: Remove unnecessary checks when setting num channels")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>