[PATCH] I2C: minor I2C doc cleanups

The I2C stack has long had "id" fields, of rather dubious utility, in
many data structures.  This removes mention of one of them from the
documentation about how to write an I2C driver, so that only drivers
that really need to use them (probably old/legacy code) will have any
reason to use this field.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>