power: supply: axp288_fuel_gauge: Added check for negative values

Variable 'pirq', which may receive negative value
in platform_get_irq().
Used as an index in a function regmap_irq_get_virq().

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Denis Arefev <arefev@swemel.ru>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>