drivers: rmnet: perf: Check if port is null on cb

If the driver callback is triggered without a valid device
then we have the potential to null ptr dereference a garbage
device when trying to get the device's port

Includes general cleanup of the register and deregister
code

Change-Id: I94a3ee7fdaff0a8f467a1f667e9a5bb3dddf032d
Signed-off-by: Conner Huff <chuff@codeaurora.org>