ACPI / PNP: Replace faulty is_hex_digit() by isxdigit()

0 is ascii for NULL. Hex digit matching should be from '0'.
Faulty version returns true for #,$,%,& etc.

Signed-off-by: Arjun Sreedharan <arjun024@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>