Update S-BUS -> PPM conversion

Actual pulse widths and S-BUS values were read from a Futaba
receiver, and a linear regression was used to determine a best fit for
the measurements.

The code and data used for the derivation can be found at
https://gist.github.com/prattmic/8857047

Basically, the S-BUS and raw PWM outputs were read from the Futaba
receiver, and fit with a linear regression.  As can be seen in the gist,
the old formula had significant error at the endpoints, a problem from
which this version does not suffer.
Remove 'sei' in interrupt

By definition, interrupts must be enabled to end up in an interrupt
handler.  Thus, enabling them is unnecessary.
Perform floating point math outside of cleared interrupts

The S-BUS to PPM conversion floating point math takes around 750us to
complete.  By doing the math when interrupts are enabled, service of the
PPM timer interrupt is delayed, occassionally enough to cause two PPM
packets to be sent back-to-back.

Perform the math while interrupts are still enabled, then simply copy
the results with interrupts disabled.