i2c: fix reference to functionality constants definition

Since commit 607ca46e ('UAPI: (Scripted) Disintegrate include/linux') the
list of functionality constants moved to include/uapi/linux/i2c.h. Update the
reference accordingly.

Fixes: 607ca46e ('UAPI: (Scripted) Disintegrate include/linux')
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>