pxa_pm_prepare() tried to validate the suspend method type. As
noted in previous commits:
eb9289eb209c372d06cee8c9c50269
the checking of the suspend type in the 'prepare' method is the
wrong place to do this; use the 'valid' method instead. This
means that pxa_pm_prepare() can be entirely removed.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>