twi: Clear TWSTO bit after STOP condition transmitted

> This change fixes a problem where a TWI driver polling the
  TWSTO bit would run on the real device but not simavr.

> More detail in for example p188 of the ATMega32A datasheet
  (rev 815D-AVR-10/2013).
Merge pull request #80 from dougszumski/avr_twi_bugfix

twi: Clear TWSTO bit after STOP condition transmitted