tcm_qla2xxx: Handle malformed wwn strings properly

If we make a variable an unsigned int and then expect it to be < 0 on
a bad character, we're going to have a bad time.  Fix the tcm_qla2xxx
code to actually notice if hex_to_bin() returns a negative variable.

This was detected by the compiler warning:

    scsi/qla2xxx/tcm_qla2xxx.c: In function ‘tcm_qla2xxx_npiv_extract_wwn’:
    scsi/qla2xxx/tcm_qla2xxx.c:148:3: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>