scsi: target: usb: gadget: Convert to new submission API

target_submit_cmd() is now only for simple drivers that do their own sync
during shutdown and do not use target_stop_session(). It will never return
a failure, so we can remove that code from the driver.

Note: Before these patches target_submit_cmd() would never return an error
for usb since it does not use target_stop_session(). If it did then we
would have hit a refcount error here:

    transport_send_check_condition_and_sense(se_cmd,
                             TCM_UNSUPPORTED_SCSI_OPCODE, 1);
    transport_generic_free_cmd(&cmd->se_cmd, 0);

transport_send_check_condition_and_sense() calls queue_status and the
driver can sometimes do transport_generic_free_cmd() from there via
uasp_status_data_cmpl(). In that case, the above
transport_generic_free_cmd() would then hit a refcount error.

So that other use of the above error path in the driver is also probably
wrong, but someone with the hardware needs to fix that.

Link: https://lore.kernel.org/r/20210227170006.5077-11-michael.christie@oracle.com
Cc: Felipe Balbi <balbi@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>