sfc: Fix loop condition for efx_filter_search() when !for_insert

efx_filter_remove_filter() fails to remove inserted filters in some cases.

For example:

  1. Two filters A and B have specifications that result in an initial
     hash collision.
  2. A is inserted first, followed by B.
  3. An attempt to remove B first succeeds, but if A is removed first
     a subsequent attempt to remove B fails.

When searching for an existing filter (!for_insert),
efx_filter_search() must always continue to the maximum search depth
for the given type rather than stopping at the first unused entry.

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