drivers: acpi: don't use own implementation of hex_to_bin()

Remove own implementation of hex_to_bin().

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>