i2c: Move i2c-dev interfaces to i2c-dev.h

Move the i2c-dev support into <linux/i2c-dev.h> where it should always
have lived.  Now <linux/i2c.h> no longer holds stuff related to the
optional userspace /dev/i2c-X interface.  Improve the descriptions
for these ioctl requests.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>