SiS warning fixes

Somehow the sis_info133 external definition ended up in libata.h and that
was included by both drivers.  However libata.h contains libata-* specific
internals and clashing defines like DRV_NAME so this makes a mess.  Move
the extern into the C file and remove the warnings

[akpm@linux-foundation.org: create sis.h to avoid extern-decl-in-C]
Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>