msm: mhi_dev: Add asynchronous read/write support in UCI driver

Add and enable (by default) support for asynchronous reads and
writes in the UCI driver. Clients can force the earlier synchronous
reads and writes by specifying the O_SYNC file flag in the file open
call.

Change-Id: I4d283c2d56e169bf7dbf01cb57b75b1911ef401d
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>