USB: pl2303: use interface device for debug

Use interface rather than usb-serial device for debugging interface
related operations. This gives more descriptive messages, such as

[  905.669436] pl2303 1-4.1:1.0: 0x40:0x1:0x8:0x0  0

rather than

[  341.943535] usb 1-4.1: 0x40:0x1:0x8:0x0  0

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>