EventHub: Associate AsociatedDevice using sysfs path, not descriptor

EventHub previously attempted to link an EventHub device to an
AssociatedDevice - which contains info about the EventHub device
obtained through sysfs - through the descriptor. Since each EventHub
device has a unique descriptor whose uniqueness is guaranteed by the
use of the nonce value, using the descriptor to link the
AssociatedDevice to device means that no two devices will share the same
AssociatedDevice even if they have the same sysfs paths. This is clearly
not working as intended.

We now link the AssociatedDevice to the EventHub device by the sysfs
path so that separate EventHub devices that share the same sysfs path
also share the same AssociatedDevice.

Unfortunately, we don't have good code coverage for reading from sysfs,
and there are no existing test cases.

Bug: 243005009
Test: Manual, check input dump
Change-Id: Iea2bb7fd0493baa6e03df15f876c5d895c997b14