scsi: libfc: pass an error pointer to fc_disc_error()

This patch is basically to silence a static checker warning.

    drivers/scsi/libfc/fc_disc.c:326 fc_disc_error()
    warn: passing a valid pointer to 'PTR_ERR'

It doesn't affect runtime because it treats -ENOMEM and a valid pointer
the same.  But the documentation says we should be passing an error
pointer.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>