hw/misc/auxbus: Fix MOT/classic I2C mode

hw/misc/auxbus: Fix MOT/classic I2C mode

Since its introduction in commit 6fc7f77fd2 i2c_start_transfer()
uses incorrectly the direction of the transfer (the last argument
is called 'is_recv'). Fix by inverting the argument, we now have
is_recv = !is_write.

Fixes: 6fc7f77fd2 ("introduce aux-bus")
Reported-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>