ds2782_battery: Get rid of magic numbers in driver_data

Constructions like 'if (id->driver_data == 1)' look quite weird.
This patch introduces 'enum ds278x_num_id', which makes things
much more understandable, i.e. 'if (id->driver_data == DS2786)'.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>