cmd_i2c.c: reduced subaddress length to 3 bytes

according to some of the comments the subaddress length is 1 or 2, but we are being
prepared for the case it becomes 3. However the code also accepted 4.
This repairs this by changing the constand 4 to 3.

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>