[elm_datetime] Corrected the logic for datetime values comparison.

[elm_datetime] Corrected the logic for datetime values comparison.

Summary:
Datetime value should be compared earlier to avoid unnecessary code execution.
Also added logic for single field comparion which will avoid to compare all fields when field limit is set.

Signed Off By: Seongjun Yim <se201.yim@samsung.com>
Signed Off By: Hyunseok Yang <hs0219.yang@samsung.com>
Signed Off By: Subodh Kumar <s7158.kumar@samsung.com>

Test Plan: NA

Reviewers: shilpasingh, raster

Reviewed By: shilpasingh, raster

Subscribers: poornima.srinivasan, govi, rajeshps

Differential Revision: https://phab.enlightenment.org/D2202