git-kernel: $(SUBDIR) should always be $(LINUX_VERSION)

Before SUBDIR was set to $(PATCHVER) which may
or may not include the minor version number of
the linux kernel version. Usually it doesn't.

So the git-clone'd linux kernel was packed without
the minor version number taken into account, which
broke further processing, as it expected the
extracted dir being named linux-$(LINUX_VERSION)
(=with minor version) rather than linux-$(PATCHVER)
(=without minor version).

Changing SUBDIR to $(LINUX_VERSION) creates
consistent behaviour here.

Signed-off-by: Mirko Vogt <mirko-openwrt@nanl.de>