hexagon/kbuild: replace CFLAGS_MODULE with KBUILD_CFLAGS_MODULE

As kbuild document & commit 6588169d says: KBUILD_CFLAGS_MODULE is
used to add arch-specific options for $(CC). From commandline,
CFLAGS_MODULE shall be used.
Doesn't have any functional change, but just follow kbuild rules.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
CC: linux-hexagon@vger.kernel.org
Acked-by: Richard Kuo <rkuo@codeaurora.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>