greybus: gb_operation: replace timeout workqueue

If an operation is issued and the response never comes back,
gb_operation_timeout() cancels the operation but never wakes up the
waiter in gb_operation_request_send().

This patch removes the timeout workqueue and changes the request wait to
wait_for_completion_interruptible_timeout(), with timeout set to
OPERATION_TIMEOUT_DEFAULT.

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