power: supply: bq27xxx: fix NULL vs 0 warnings

The driver has a lot of sparse warnings for using 0 as a NULL
pointer when NULL would be appropriate. Change the 0 values
to NULL to fix the warnings, some of which are shown here:

drivers/power/supply/bq27xxx_battery.c:984:23: warning: Using plain integer as NULL pointer
drivers/power/supply/bq27xxx_battery.c:985:23: warning: Using plain integer as NULL pointer
drivers/power/supply/bq27xxx_battery.c:986:23: warning: Using plain integer as NULL pointer
drivers/power/supply/bq27xxx_battery.c:987:23: warning: Using plain integer as NULL pointer
drivers/power/supply/bq27xxx_battery.c:988:23: warning: Using plain integer as NULL pointer

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>