power: supply: axp288_charger: Fix unchecked return value

Currently there is no check on platform_get_irq() return value
in case it fails, hence never actually reporting any errors and
causing unexpected behavior when using such value as argument
for function regmap_irq_get_virq().

Fix this by adding a proper check, a message reporting any errors
and returning *pirq*

Addresses-Coverity-ID: 1443940 ("Improper use of negative value")
Fixes: 843735b7 ("power: axp288_charger: axp288 charger driver")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>