i2c-dev: Cleanups

* We no more need to include platform_device.h.
* Delete the to_i2c_dev macro, which is no more used (and no more valid
  either.) 
* Drop i2c_dev.minor. Now that the minor number always matches the i2c
  adapter number, this field is redundant with i2c_dev.adap->nr.
* Delete i2c_dev_get_by_adapter() which is now redundant with
  i2c_dev_get_by_minor() for the same reason.
* Drop the local variable dev in i2cdev_attach_adapter(), we can
  easily do without it.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>