ACPI: scan: initialize local variable to avoid garbage being returned

In the unlikely event that there are no callback calls made in
acpi_walk_dep_device_list(), local variable ret will be returned as
an uninitialized value.

Clean up static analysis warnings by ensuring ret is initialized.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: a9e10e58 ("ACPI: scan: Extend acpi_walk_dep_device_list()")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Daniel Scally <djrscally@gmail.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>