kbuild: deb-pkg: call more misc debhelper commands

Use dh_prep instead of removing old build directories manually.

Use dh_clean instead of removing build directories and debian/files
manually.

Call dh_testdir and dh_testroot for preliminary checks.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>