scsi_transport_srp: Fix kernel-doc warnings

The following command has been used to verify that the kernel-doc
tool no longer complains about undocumented fields:

    scripts/kernel-doc -html drivers/scsi/scsi_transport_srp.c \
      include/scsi/scsi_transport_srp.h >srp-transport-doc.html

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Sebastian Riemer <sebastian.riemer@profitbricks.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>