sfc: Remove initialisation of RX_FILTER_CTL_REG.NUM_KER

We have long since given up doing RSS on Falcon A1 and therefore we
would always write the default value of 0.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>