brcompatd: Fix handle_fdb_query_cmd() return value on error.

brcompatd: Fix handle_fdb_query_cmd() return value on error.

handle_fdb_query_cmd() should return an error when an error occurs, but
in this case it was always returning 0, because error is known to have
value 0 at this point.

Nothing really uses the return value here, and so eventually it would make
sense to just change the return type here and in the rest of the
handle_*() functions to void.