builddeb: clean generated package content

For each binary Debian package, a directory with the package name is
created in the debian directory. Correct the generated file matches in the
package's clean target, which were renamed without adjusting the target.

Fixes: 1694e94e ("builddeb: match temporary directory name to the package name")
Signed-off-by: Bastian Germann <bage@linutronix.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>