msm: mhi_dev: Add QMAP flow control hardware channel

Add hardware channels used to pass control messages for
QMAP flow control. Also update the maximum number of
channels supported in MHI and remove unused channel
enum name.

Change-Id: Iaa0778400cebb4b65e184451b7026053ba9e6524
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>