msm: ion: fix ion cache maintenance issue

ion cache maintenance code while doing cache maintenance
on each sg is only considering page length which may not
always be correct. An sg node can have more than page
length contiguous pages.

Update the code to properly take care of any length sg.

Change-Id: Ied380fa32cbda5384bab8ec2625ceaf1010fd314
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>