power: supply: bq24735: allow polling even if there is no ac-detect gpio

It is possible to verify AC adapter presence via a register read, without
any physical connection to the ACOK pin on the charger. Allow this.

Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>