greybus: always drop reference in gb_operation_work()

Currently we issue a warning in gb_operation_work() if an operation
has no callback function defined.  But we return without dropping
the reference to the operation as we should.

Stop warning if there's no callback, call it only if it's defined,
and always drop the operation reference before returning.

This means we're now treating a NULL callback pointer as a normal
condition.

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