ads1298_driver.ino : rearrange functions to get this to compile

Tried to verify (not upload yet) ads1298_driver.ino in arduino 1.5.5, 
got undeclared function errors, and rearranged functions, 
such that all functions are defined before they are called. 

This is strictly copy&paste, so there should be no functional difference 
(except that my version actually compiles)

Signed-off-by: Christoph Maier <cm.hardware.software.elsewhere@gmail.com>