Fix I2C endTransmission(false) with USCI

Fixes Wire.endTransmission(false) for MSP430G2553 (USCI). This will
allow to read from I2C devices with restart condition.

For example:
Wire.beginTransmission(i2caddress);
Wire.write(addr);
Wire.endTransmission(false);
Wire.requestFrom(i2caddress, 1);
return Wire.read();

See http://forum.43oh.com/topic/3154-i2c-issues/#entry34292