Check for illegal TWI bus frequency setting

[mikaelpatel]

Improve TWI::set_freq() https://github.com/mikaelpatel/Cosa/blob/master/cores/cosa/Cosa/TWI.hh#L296; do not allow higher frequency than the Master can handle.

See also arduino/Arduino#1912 (comment).

[mikaelpatel]

TWI::Driver could set max frequency in constructor. The bus max frequency would be the min of the TWI::Driver's in the sketch.

[mikaelpatel]

See commit 0f8841b.