iscsi-target: make one-bit bitfields unsigned

Signed bitfields are a problem because instead of being 1 or 0 like
you'd expect they are 0 and -1.  It doesn't cause a problem in this case
but sparse complains:

drivers/target/iscsi/iscsi_target_core.h:564:56: error: dubious one-bit
signed bitfield

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>