iscsi-target: use native hex2bin for chap_string_to_hex

This patch converts chap_string_to_hex() to use hex2bin() instead of
the internal chap_asciihex_to_binaryhex().

(nab: Fix up minor compile breakage + typo)

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>