This commit fixes the mistake introduced in the commit 34fadb685905
('commands: i2c_write: enable raw write to address').
The intended behaviour for i2c_write command is
"use raw i2c write only if reg address is not defined".
Before the commit 34fadb685905 reg address == 0
was acceptable. After the commit 34fadb685905 reg address == 0
became unacceptable ("undefined") and will lead to raw i2c write.

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