staging: ath6kl: use native methods from kernel library

There are already implemented methods such hex_to_bin() or isxdigit() in
the kernel. Let's use them.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Vipin Mehta <Vipin.Mehta@Atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>