PLDM: Handling modified PDRs

This commit handles the PDRs which are modified.
This also prevents the looping of host PDRs when we get
the modified PDR in between the PDR exchange. With this
change we will fetch only the modified PDRs and not all
the PDRs again.

Change-Id: If674f23ce83f44853c74139c9d3c4e5aea05259a
Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>