scsi: target: Make iscsit_register_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/20200803150008.83920-1-maxg@mellanox.com


Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>