kbuild: deb-pkg: do not take KERNELRELEASE from the source version

KERNELRELEASE does not need to match the package version in changelog.
Rather, it conventially matches what is called 'ABINAME', which is a
part of the binary package names.

Both are the same by default, but the former might be overridden by
KDEB_PKGVERSION. In this case, the resulting package would not boot
because /lib/modules/$(uname -r) does not point the module directory.

Partially revert 3ab18a62 ("kbuild: deb-pkg: improve the usability
of source package").

Reported-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Fixes: 3ab18a62 ("kbuild: deb-pkg: improve the usability of source package")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>