greybus: cancel whole operation on interrupt

Cancel the operation--not just the request message--if waiting
for a synchronous operation to complete is interrupted.  Return
the operation result (which in that case will be -EINTR).  The
cancelation will result in the normal operation completion path
being taken before returning.

Make gb_operation_wait() private, since it's only ever used for
for synchronous operations.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>