isdn: remove ishexdigit() in regard to isxdigit()

Samll cleanup in drivers/isdn/gigaset/capi.c where own implementation of
isxdigit() has been changed to kernel native one.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Acked-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>