mhi_dev: mhi: update variable type

Update variable type such that value can be accomodated
based on the platform.

Change-Id: I2c69e1a242caaddc8d9e300199a763ee92777bd4
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>
Signed-off-by: Chandra Devireddy <cdevired@codeaurora.org>