msm: mhi: Remove incorrect iomem operations

Remove the incorrect direct de-reference of device mapped
IO mememory and replace with kernel standard handlers.

CRs-Fixed: 728285
Change-Id: I0b3afe8a62369057b4a6b45b1a71bd9f5a52ce1d
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>