linux: Change commit to master branch

I'm planning to organize the sm6125-mainline project with the following
branch scheme:

- master: Stable release, commits here will be sent to LKML
- upstream: For pulling changes from torvalds/linux, testing and fixing
  prior to merging into master
- Any other branches will be used to implement new features to master

This commit switches the previously used commit in favor of the master branch.

linux-postmarketos-qcom-sm6125/APKBUILD

@@ -23,7 +23,7 @@ makedepends="
 
 # Source
 _repository="linux"
-_commit="ffe21073175927429b89357ce1fede7eb0d27efe"
+_commit="master"
 _config="config-$_flavor.$arch"
 source="
 	$pkgname-$_commit.tar.gz::https://gitlab.com/sm6125-mainline/$_repository/-/archive/$_commit.tar.gz