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>

Documentation/i2c/functionality

@@ -12,7 +12,7 @@ FUNCTIONALITY CONSTANTS
 -----------------------
 
 For the most up-to-date list of functionality constants, please check
-<linux/i2c.h>!
+<uapi/linux/i2c.h>!
 
   I2C_FUNC_I2C                    Plain i2c-level commands (Pure SMBus
                                   adapters typically can not do these)