scsi: iscsi: Make iscsi_unregister_transport() return void

This function always returns 0. We can make it return void to simplify the
code. Also, no caller ever checks the return value of this function.

Link: https://lore.kernel.org/r/20220616080210.18531-1-mgurtovoy@nvidia.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>