fixes bug in regional bc time calculation in fv_tracer2d.F90

[bensonr]

Description

Fixes a bug identified during the merge into dev/emc. The bug affects the time trigger for the regional bc updates.

Fixes # (issue)

How Has This Been Tested?

Tested via dev/emc merge.

Checklist:

Please check all whether they apply or not

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[lharris4]

The fix now gives the correct behavior. Earlier behavior would act to greatly extend the extrapolation, leading to potentially very large values of the incoming tracer (or perhaps even negatives), leading to artifacts and instability.