fix interrupt based i2c

typically UCB0IV is read out and compared using switch-case structure, however because of contiki switch-case fear I used simple if - elseif, however notice that UCB0IV was read out every if elseif block, therefore it only had correct value in first block, all other blocks read out 0x0000
clean up i2c module

* remove polling based receive
* use contiki coding style
now only interrupt based i2c rx is used