iscsi-target: Fix hex2bin warn_unused compile message

Fix the following compile warning with hex2bin() usage:

drivers/target/iscsi/iscsi_target_auth.c: In function ‘chap_string_to_hex’:
drivers/target/iscsi/iscsi_target_auth.c:35: warning: ignoring return value of ‘hex2bin’, declared with attribute warn_unused_result

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>