net: qualcomm: rmnet: Fix use after free in uplink aggregation

On device unregistration, the transmit aggregation worker thread
is never cancelled, so any instance of it scheduled at a later
point of time would be a use after free.

CRs-Fixed: 2345437
Change-Id: I568c5f7035f637502c349fd239234f5b7cfe9783
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>