i2c: Refactor a kfree in i2c-dev

Refactor kfree(i2c_dev) into return_i2c_dev(). This saves some
code and makes more sense, as the memory is allocated in
get_free_i2c_dev().

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Greg Kroah-Hartman <gregkh@suse.de>