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 8db182f4a8a6e2dcb8b65905ea4af56210e65430
('sfc: Remove now-unused filter function').

Shut the compiler up by initialising it to 0.

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

drivers/net/ethernet/sfc/filter.c

@@ -662,7 +662,7 @@ s32 efx_filter_insert_filter(struct efx_nic *efx, struct efx_filter_spec *spec,
 	struct efx_filter_table *table = efx_filter_spec_table(state, spec);
 	struct efx_filter_spec *saved_spec;
 	efx_oword_t filter;
-	unsigned int filter_idx, depth;
+	unsigned int filter_idx, depth = 0;
 	u32 key;
 	int rc;