power: axp288_charger: fix for API change

Caused by commit 843735b7 ("power: axp288_charger: axp288 charger
driver") from the battery tree interacting with commit 046050f6
("extcon: Update the prototype of extcon_register_notifier() with enum
extcon") from the extcon tree.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>