HID: usbhid: initialize interface pointers early enough

Move the initialization of USB interface pointers from _start()
over to _probe() callback, which is where it belongs.

This fixes case where interface is NULL when parsing of report
descriptor fails.

LKML-Reference: <20100213135720.603e5f64@neptune.home>
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>