HID: mcp2221: Set ACPI companion

In scenarios where an I2C device tree is defined in ACPI and exists off
the MCP2221 I2C bus, the devices could not be instantiated.
Mark the USB port that the MCP2221 is connected to as its ACPI companion
so that the USB device can be bound to the ACPI tree when enumerated.
With this change the downstream I2C tree devices can be instantiated on
ACPI systems.

Signed-off-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>