power: bq27xxx_battery: fix signedness bug in bq27xxx_battery_read_health()

We need flags to be signed for the error handling to work.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576



Fixes: 74aab849 ('power: bq27xxx_battery: Cleanup health checking')
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-By: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>