mhi: core: Prevent MHI reg read upon endpoint crash

MHI reset is triggered as part of EP shutdown. Shutdown
is blocked until MHI RESET bit is cleared in MHI CTRL reg.
There is a possibility of PCIe link going down during this
time. Since MHI RESET bit never gets cleared after 2 sec
wait_event_timeout reads the MHI CTRL reg and this results
into a memory abort. Fix this issue by using a boolean flag
as condition in wait_event_timeout. This flag is set when
MHI reset is triggered and cleared when reset is done.

Change-Id: I5b1831518255a38cde5b60406b5b9e9386b5477e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>