power: supply: cpcap-charger: Fix null check for power_supply_get_by_name

In the Linux kernel, the following vulnerability has been resolved:

power: supply: cpcap-charger: Fix null check for power_supply_get_by_name

In the cpcap_usb_detect() function, the power_supply_get_by_name()

function may return `NULL` instead of an error pointer.

To prevent potential null pointer dereferences, Added a null check.