mhi: core: Set correct DMA mask on device

Set the correct DMA mask on the platform device to
avoid bounce buffering from the default 32 bit mask.

Change-Id: I7e201e83fd21b0f0a6f57754a3b6f3eded9be58f
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>
Signed-off-by: Tony Truong <truong@codeaurora.org>