power: supply: cpcap-charger: fix incorrect return value check

Function platform_get_irq_byname() returns a negative error code on
failure, and a zero or positive number on success. However, in function
cpcap_usb_init_irq(), positive IRQ numbers are also taken as error
cases. Use "if (irq < 0)" instead of "if (!irq)" to validate the return
value of platform_get_irq_byname().

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>