pw_kvs: Fix checksum finish usage
pw_kvs: Fix checksum finish usage

When KVS calculates a checksum, it does not always call finish. This
change ensures finish is always called when calculating a checksum.

Change-Id: I4898735835e5838c8a5a98e5f738f07346b1059f

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed