Fixed index offset in mask port mapping.

[jamesdunn]

Fixing index offset in mask port mapping. This was originally done in #34 but did not make it to master.

Failure looked like

Exception in thread "main" firrtl.passes.PassExceptions:
firrtl.passes.CheckWidths$BitsWidthException: : [module tag_array_ext] High bit 9 in bits operator is larger than input width 8 in bits(R│[dunn@bwrcrdsl-1] /tools/B/dunn/hammer-workspace/project-template
W0_mask, 9, 9).

[edwardcwang]

Do you have a unit test for this?

[jamesdunn]

Do you have a unit test for this?

Not yet. It resolved the error I was seeing with the ST28 mems in EE194, but I can come up with a unit test.

[jwright6323]

@jamesdunn @edwardcwang I'm going to file a separate issue for the unit tests and merge this now.