[roll pigweed] pw_kvs: Clean up in FlashPartition

Minor clean up in FlashPartition, in prep for adding a new type
of FlashPartition.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/106953

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 42208d1596c0217..10d8429fd55730e
Roller-URL: https://ci.chromium.org/b/8805017711168257649
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib336cef81f247a7c40e4ea0db136923c6b249bb9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/107960
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index 42208d1..10d8429 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 42208d1596c0217d0e7adbb888094c976fc47b0e
+Subproject commit 10d8429fd55730eb82590b3478a3d2bc4e7d3126