USI_TWI::endTransmission() not returning status

In TinyWireM.cpp the function endTransmission() is not returning proper status.
This breaks code that checks for return status information using the endTransmission() function.
This:
```
uint8_t USI_TWI::endTransmission() {
endTransmission(1);
}
```
Should be changed to:
```
uint8_t USI_TWI::endTransmission() {
return(endTransmission(1));
}
```

Open TinyWireM.cpp and read USI_TWI::endTransmission(), then compare it with the overloaded endTransmission(1) call shown in the issue. Done means the no-argument function propagates the return status so callers checking endTransmission() receive the proper result.

Written by the indexing model from the issue text.