power: supply: bq27xxx: fix __be16 warnings

The bq27xxx_dm_reg_ptr() should return a __be16 as the result
is being passed to be16_to_cpup() to convert to the proper
cpu endian value. Move to using __be16 as appropriate to fix
the following sparse warnings:

drivers/power/supply/bq27xxx_battery.c:1293:26: warning: incorrect type in argument 1 (different base types)
drivers/power/supply/bq27xxx_battery.c:1293:26:    expected restricted __be16 const [usertype] *p
drivers/power/supply/bq27xxx_battery.c:1293:26:    got unsigned short [usertype] *prev
drivers/power/supply/bq27xxx_battery.c:1304:17: warning: incorrect type in argument 1 (different base types)
drivers/power/supply/bq27xxx_battery.c:1304:17:    expected restricted __be16 const [usertype] *p
drivers/power/supply/bq27xxx_battery.c:1304:17:    got unsigned short [usertype] *prev
drivers/power/supply/bq27xxx_battery.c:1316:15: warning: incorrect type in assignment (different base types)
drivers/power/supply/bq27xxx_battery.c:1316:15:    expected unsigned short [usertype]
drivers/power/supply/bq27xxx_battery.c:1316:15:    got restricted __be16 [usertype]

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