aux: fix break that wanted to break two levels out

The last "ret = AUX_I2C_NACK;" is dead, because it is always overridden
by AUX_I2C_ACK.  What really the code wants is to jump out of the switch
statement, and a "return" will not cut it because it would omit a debug
printf.

Change the logic so that we can break out of the while loop.  For clarity,
hoist the bus->last_* assignments up, right after i2c_start_transfer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
