drivers: qcom: rpmh: Use wait_event instead of wait_event_interruptible

wait_event_interruptible() breaks out of wait for all sorts of signals,
which is undesirable here. wait_event() waits for the completion events
that we want.

Reboot issues a signal to all waiting threads and once the wait is
complete, we wind up the stack and the RPMH message objects that were
allocated in the stack are no longer valid. wait_event() solves this
issue by continuing to wait, until the completion signal is received.

Change-Id: I3a0e8da5a317a4bc5c0599c30aa2f745890259db
Signed-off-by: Lina Iyer <ilina@codeaurora.org>