When continuously running wifi up/down sequence, the napi poll
can be scheduled after the CE buffers being freed by ath10k_pci_flush

Steps:
  In a certain condition, during wifi down below scenario might occur.

ath10k_stop->ath10k_hif_stop->napi_schedule->ath10k_pci_flush->napi_poll(napi_synchronize).

In the above scenario, CE buffer entries will be freed up and become NULL in
ath10k_pci_flush. And the napi_poll has been invoked after the flush process
and it will try to get the skb from the CE buffer entry and perform some action on that.
Since the CE buffer already cleaned by pci flush this action will create NULL
pointer dereference and trigger below kernel panic.

Unable to handle kernel NULL pointer dereference at virtual address 0000005c
PC is at ath10k_pci_htt_rx_cb+0x64/0x3ec [ath10k_pci]
ath10k_pci_htt_rx_cb [ath10k_pci]
ath10k_ce_per_engine_service+0x74/0xc4 [ath10k_pci]
ath10k_ce_per_engine_service [ath10k_pci]
ath10k_ce_per_engine_service_any+0x74/0x80 [ath10k_pci]
ath10k_ce_per_engine_service_any [ath10k_pci]
ath10k_pci_napi_poll+0x48/0xec [ath10k_pci]
ath10k_pci_napi_poll [ath10k_pci]
net_rx_action+0xac/0x160
net_rx_action
__do_softirq+0xdc/0x208
__do_softirq
irq_exit+0x84/0xe0
irq_exit
__handle_domain_irq+0x80/0xa0
__handle_domain_irq
gic_handle_irq+0x38/0x5c
gic_handle_irq
__irq_usr+0x44/0x60

Tested on QCA4019 and firmware version 10.4.3.2.1.1-00010
Signed-off-by: Tamizh chelvam <tamizhr@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>