greybus: operation: allow drivers to define custom timeouts

Add new interface gb_operation_request_send_sync_timeout, which allows
drivers to define a custom operation timeout instead of the default
one-second timeout.

The timeout is expected to depend on protocol and operation and
therefore needs to be configurable.

Note that that a timeout of zero is used to wait indefinitely.

Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>