[SCSI] srp.h: Add I/O Class values

Add enum values for I/O Class values from rev. 10 and rev. 16a SRP
drafts.  The values are used to detect targets that implement obsolete
revisions of SRP, so that the initiator can use the old format for
port identifier when connecting to them.

Signed-off-by: Ramachandra K <rkuchimanchi@silverstorm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>