power: supply: ab8500_fg: silence uninitialized variable warnings

If kstrtoul() fails then we print "charge_full" when it's uninitialized.
The debug printk doesn't add anything so I deleted it and cleaned these
two functions up a bit.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>