sfc: Work around bogus 'uninitialised variable' warning

With some gcc versions & optimisations, the compiler will warn that
'depth' in efx_filter_insert_filter() may be used without being
initialised, although this is not the case.

This is related to inlining of efx_filter_search(), which only has
one caller since commit 8db182f4


('sfc: Remove now-unused filter function').

Shut the compiler up by initialising it to 0.

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