target: simplify target_parse_naa_6h_vendor_specific()

This patch adds a minor simplfication in target_parse_naa_6h_vendor_specific()
to remove direct isxdigit() + ctype.h usage.

(nab: Fix next assignment breakage in for loop)

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