c6x/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: Mark Salter <msalter@redhat.com>
CC: Aurelien Jacquiot <jacquiot.aurelien@gmail.com>
CC: linux-c6x-dev@linux-c6x.org
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>