i2c-dev: Fix I2C_SLAVE ioctl comment

The first part of the comment is wrong since November 2007, delete it.

The second part of the comment is related to I2C_PEC, not I2C_SLAVE, so
move it where it belongs.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>