IB/srpt: Document all structure members in ib_srpt.h

This patch avoids that the following command reports any warnings:

scripts/kernel-doc -none drivers/infiniband/ulp/srpt/ib_srpt.h

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>