debian-meta/etc/kernel/postinst.d/reform-qcacld2: abort running postinst hook if reform-qcacld2-*-mnt-reform-arm64 is not installed

This script lives in /etc and is as such a config file and will not get removed (unless with --purge) when the user uninstalls the reform-qcacld2 package. To avoid re-installing reform-qcacld2 even though the user has removed it, check here if the kernel module provided by the package reform-qcacld2-*-mnt-reform-arm64 is actually installed. If not, we do not need to and should not proceed.

These issues were discovered when debugging with BoostisBetter. This MR tries to make sure others do not run into the same problems.