lvol: restore the channel operation number into 512

We would still like to keep up this Macro, then if users
want to change this value with their own ones,
they can change it.

Change-Id: Ic13c9ba0889955cc41a75f700c58822728782f6c
Signed-off-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-on: https://review.gerrithub.io/394311


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>