Sometimes for communication with a simple I2C devices
(e.g. PCF8574 or TM1650) it's necessary to send only
one data byte into the I2C device.
Current i2c_write command makes this impossible because
you can't just pass 'device address' and 'register number'
(or 'device address' and 'one data byte') to the command.
You always have to pass all three parameters:
'device address', 'register number' and 'data'.

This commit fixes the problem.

Sample usage:

  barebox@barebox sandbox:/ i2c_write -a 0x24 0x01

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>