drivers: rmnet_shs: Remove rmnet ep access

Rmnet driver allocates rmnet_endpoint which rmnet_shs was using
to keep track of endpoints that needed. However rmnet driver
frees the memory before endpoint unregistration so this leaves a
potential race condition where the wq can run after freeing.

Change is to instead use net_dev refrerences we keep track of from
net_dev_cb and no longer use rmnet_endpoints allocated by rmnet driver.
Rmnet_shs was only using netdev references in rmnet_endpoint so no
impact should be expected.

This use-after-free would cause the following crash-signature.

<6> Unable to handle kernel paging request at virtual address 00005000
<6> Mem abort info:
<6>   Exception class = DABT (current EL), IL = 32 bits
<6>   SET = 0, FnV = 0
<6>   EA = 0, S1PTW = 0
<6>   FSC = 5
<6> Data abort info:
<6>   ISV = 0, ISS = 0x00000005
<6>   CM = 0, WnR = 0
<6> user pgtable: 4k pages, 39-bit VAs, pgd = 0000000070b0b425
<6> Internal error: Oops: 96000005 [#1] PREEMPT SMP
<6> Workqueue: rmnet_shs_wq rmnet_shs_wq_process_wq [rmnet_shs]
<6> task: 00000000deaad59d task.stack: 00000000053e0949
<2> pc : rmnet_shs_wq_update_ep_rps_msk+0x3c/0xd8 [rmnet_shs]
<2> lr : rmnet_shs_wq_update_ep_rps_msk+0x28/0xd8 [rmnet_shs]
<2> Call trace:
<2>  rmnet_shs_wq_update_ep_rps_msk+0x3c/0xd8 [rmnet_shs]
<2>  rmnet_shs_wq_update_stats+0x98/0x928 [rmnet_shs]
<2>  rmnet_shs_wq_process_wq+0x10c/0x248 [rmnet_shs]
<2>  process_one_work+0x1f0/0x458
<2>  worker_thread+0x2ec/0x450
<2>  kthread+0x11c/0x130
<2>  ret_from_fork+0x10/0x1c

CRs-Fixed: 2541604
Change-Id: I7026f2564c463f4ca989af97572e2a8fe5652087
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>