NetworkProtocol: IPMI readonly if no RMCP

Return PropertyNotWritable when trying to modify
IPMI/ProtocolEnabled through
/redfish/v1/Managers/bmc/NetworkProtocol/ on systems built with
phosphor-no-ipmi-rmcp.

Tested: curl -k \
-H 'Content-Type: application/json' \
-X PATCH \
https://$bmc/redfish/v1/Managers/bmc/NetworkProtocol/ \
-d '{"IPMI":{"ProtocolEnabled":true}}'
Responded with "Base.1.19.PropertyNotWritable" for IPMI/ProtocolEnabled

Change-Id: I9edf0d0c7bea152a7df60176e377293dc17ff33b
Signed-off-by: Crystal Huang <crzhuang1@gmail.com>