initrd: cmdline-reader: fix setting uint properties

Previously a uint property was assigned with a guint64 value, which
has a different size. Fix this and add a warning when the read value
can't be converted.

Fixes: ecc074b2

#66
4 jobs for bg/issue66 in 38 minutes and 32 seconds (queued for 27 minutes and 59 seconds)
Status Job ID Name Coverage
  Test
passed #32579
allowed to fail
checkpatch

00:05:07

passed #32580
f28_build_default

00:21:09

passed #32581
f28_rpm_autotools_dbg

00:14:25

passed #32582
f28_rpm_meson_dbg

00:10:37