i2c: have I2C receive operation return uint8_t

i2c: have I2C receive operation return uint8_t

It is never supposed to fail and cannot return an error, so just
have it return the proper type.  Have it return 0xff on nothing
available, since that's what would happen on a real bus.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>