Fix Icom IC-756 Pro III get data mode.

The "1A 06" command to get the data mode returns 2 bytes rather than
the one byte documented in the user manual.

Fix indexing error, collect data mode from correct reply byte.
Merge IC-756 Pro III fix from Bill, G4WJS

Merge branch 'fix_icom_ic756p3_data_mode' of git://git.code.sf.net/u/bsomervi/hamlib