hw/i2c: Make i2c_start_transfer() direction argument a boolean

hw/i2c: Make i2c_start_transfer() direction argument a boolean

Make the argument representing the direction of the transfer a
boolean type.
Rename the boolean argument as 'is_recv' to match i2c_recv_send().
Document the function prototype.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <20200621145235.9E241745712@zero.eik.bme.hu>
[PMD: Split patch, added docstring]
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>