[cooja/mspsim] Msp802154Radio: Fix maximum power indicator

Previously getOutputPowerIndicatorMax() returned the fixed value 31.
This is valid for e.g. the mspsim CC2420 radio implementation but not for the CC2520 implementation where the maximum returned value is 9.

Thus to fix transmission range issues (for example for Wismote node) the maxium value provided by the radio implementaiton must be used.
Merge pull request #799 from ejoerns/pull-req/cooja-mspsim-radiofix

[Cooja/MSPSim] Msp802154Radio: Fix maximum power indicator