cris: kgdb: use native hex2bin

There are kernel native helpers to convert hex ascii to the binary format:
hex_to_bin() and hex2bin(). Thus, no need to reimplement them customly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jesper Nilsson <jespern@axis.com>