[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:
	eb9289eb20
	9c372d06ce
	e8c9c50269

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>

arch/arm/common/sharpsl_pm.c

@@ -766,7 +766,6 @@ static void sharpsl_apm_get_power_status(struct apm_power_info *info)
 }
 
 static struct pm_ops sharpsl_pm_ops = {
-	.prepare	= pxa_pm_prepare,
 	.enter		= corgi_pxa_pm_enter,
 	.valid		= pm_valid_only_mem,
 };