XTENSA: static should be at beginning of declaration

Make sure that the 'static' keywork is at the beginning of declaration
for arch/xtensa/variants/s6000/include/variant/dmac.h

This gets rid of warnings like
  warning: ‘static’ is not at beginning of declaration
when building with -Wold-style-declaration (and/or -Wextra which also
enables it).

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>