sfc: farch: fix compile warning in efx_farch_dimension_resources()

Fix the following kernel build warning when CONFIG_SFC_SRIOV is disabled:

  drivers/net/ethernet/sfc/farch.c: In function ‘efx_farch_dimension_resources’:
  drivers/net/ethernet/sfc/farch.c:1671:21: warning: variable ‘buftbl_min’ set but not used [-Wunused-but-set-variable]
    unsigned vi_count, buftbl_min, total_tx_channels;

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>