[ARM] pxa: Fix PXA27x suspend type validation, remove pxa_pm_prepare()

pxa_pm_prepare() tried to validate the suspend method type.  As
noted in previous commits:
	eb9289eb
	9c372d06
	e8c9c502

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>