drivers: rmnet_shs: Fix ep down race condition

Previously when a vnd would go down we would clear it's struct
in netdev_notifier_cb context. However if rmnet_shs_wq is running
it can get into a state where the struct has been cleared but has
passed error checking. Resulting in a use after clear error of our own
internal ep structs.

This error can cause the following stack error.

<6> Unable to handle kernel NULL pointer dereference at virtual address 00000328
<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 = 0000000071c11f76
<6> [0000000000000328] *pgd=0000000000000000, *pud=0000000000000000
<6> Internal error: Oops: 96000005 [#1] PREEMPT SMP
<2> pc : rmnet_shs_wq_update_ep_rps_msk+0x24/0xb0 [rmnet_shs]
<2> lr : rmnet_shs_wq_update_ep_rps_msk+0x1c/0xb0 [rmnet_shs]
<2> Call trace:
<2>  rmnet_shs_wq_update_ep_rps_msk+0x24/0xb0 [rmnet_shs]
<2>  rmnet_shs_wq_refresh_ep_masks+0x3c/0x54 [rmnet_shs]
<2>  rmnet_shs_wq_process_wq+0x140/0x83c [rmnet_shs]
<2>  process_one_work+0x1e0/0x410
<2>  worker_thread+0x27c/0x38c
<2>  kthread+0x12c/0x13c
<2>  ret_from_fork+0x10/0x18
<6> Code: b40003b3 94000900 f9400a68 f9400508 (f9419508)

Change-Id: Ic1529b0e2645df08432c1ba22821db68d1c58951
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>