soc: qcom: spcom: fix rx path to support power collapse (cxpc)

This change replaces wait_for_completion() with
wait_for_completion_interruptible() variant for rx path.
This make possible for user process context to remain
TASK_INTERRUPTIBLE and receive signal during system suspend.
read() syscall returns -ERESTART to trigger repeat of the call.

Change-Id: Id469d606aaa900dc8f69a509d35dd6094723b031
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>