fixing up some default values.

The ACK timeout value was measured using the RadioToBlink test app
to be optimally around 2.5ms. I think I might be measuring it slightly
differently than the Soft-ACK rfxlink layer. I like my way better at
the moment and it's not super important to be consistent here.

I think our 2.5ms is approximately equal to the CC2420 stack's
800uS.

Also fixed the Node's default channel.