mhi: core: Add range check for channel id received in event ring

The mhi_process_cmd_completion function reads cmd channel id from
cmd_pkt using MHI_TRE_GET_CHID, the value is under the control of MHI
devices and can be any value between 0 and 255. However the max channel
is defined in device tree file and it is usually smaller than 255. This
can cause out of bound access to the channel array. Fix this by checking
the channel id received in cmd ring against the max channel allowed on
target.

Change-Id: Ib6faf67c7eae67186b3a44e6b1612deff6bf05fa
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>