ds2760_battery: delay power supply registration

This fixes a race condition I recently introduced with the PMOD feature
addition (cef437e3: "w1: ds2760_battery: add support for sleep mode
feature").

Postpone the call to power_supply_register() to fix it.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Szabolcs Gyurko <szabolcs.gyurko@tlt.hu>
Acked-by: Matt Reimer <mreimer@vpop.net>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>