net: qualcomm: rmnet: Remove locking from dowlink marker

All downlink marker operations occur in sequence - module
init on bootup and marker list parse during transfer
time. Remove this unnecessary locking for now.

CRs-Fixed: 2233026
Change-Id: I881a3e7f594218f9068dbe4037fe553a82b637d3
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>