ACPI / bind: Redefine acpi_get_child()

Since acpi_get_child() is the only user of acpi_find_child() now,
drop the static inline definition of the former and redefine the
latter as new acpi_get_child().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Tested-by: Aaron Lu <aaron.lu@intel.com> # for ATA binding