soc: qcom: spcom: return EINTR on wait interrupted

wait_for_completion_interruptible() returns ERESTARTSYS when
the device goes to low power mode.
The SPU subsystem is shutdown on deep low power mode,
so the interrupted operation is not recoverable.
return EINTR to user space to propagate the error to the user app.

Change-Id: I3b47b971d4411f977a0f68c1ce838fd92198c75d
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>