Other functions from I2C slave API are named "i2c_slave_XXX()".
Follow that pattern with set_address(). Add docstring along.
No logical change.

Patch created mechanically using:

  $ sed -i s/i2c_set_slave_address/i2c_slave_set_address/ \
    $(git grep -l i2c_set_slave_address)

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>