sfc: Refactor Falcon-arch search limit reset

Currently every call to efx_farch_filter_table_clear_entry() is
shortly followed by a conditional reset of the table limits.  The new
limits (0) are not pushed to hardware until the next filter insertion.
Move both the reset and the hardware reconfiguration into
efx_farch_filter_table_clear_entry(), and add an explanatory comment.

Also, make consistent use of the term 'search limit' for the maximum
number of probes the NIC must make when searching for a filter of a
particular type.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>