Input: make i2c device ids constant

The id_table field of the struct i2c_driver is defined as constant
in <linux/i2c.h> so it makes sense to mark the initialization data also
constant.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>