sfc: fix build without CONFIG_RFS_ACCEL

The rfs members of struct efx_channel are under CONFIG_RFS_ACCEL.
Ethtool stats which access those need to be as well.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: ca70bd42 ("sfc: add statistics for ARFS")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>