i2c: Merge i2c_attach_client into i2c_new_device

Now that i2c_attach_client is no longer exported, it doesn't need to
be a separate function. Merge it into its only user, i2c_new_device.

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