i2c-sibyte: Fix an error path

If the registration of the second I2C channel fails, we really want to
unregister the first one before we return with an error.

While we're here, fix the printk right above so that it displays the
real driver name.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Ralf Baechle <ralf@linux-mips.org>