[MTD] physmap: Fix suspend/resume/shutdown bugs.

Don't call suspend/resume functions if they have not been
defined.

Signed-off-by: Robert Jarzmik <rjarzmik@free.fr>
Acked-By: Jörn Engel <joern@logfs.org>
Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>