greybus: gb_operation: drop operation refcount on cancel

An extra reference is taken out on an operation in
gb_operation_request_send(). If the response never arrives, we need to
put back the reference.

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