hpsa: fix bad comparison of signed with unsigned in hpsa_update_scsi_devices

rescan_hba_mode was defined as a u8 so could never be less than zero:

        rescan_hba_mode = hpsa_hba_mode_enabled(h);
        if (rescan_hba_mode < 0)
                goto out;

Signed-off-by: Joe Handzik <joseph.t.handzik@hp.com>
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>