ath10k: Avoid potential page alloc BUG_ON in tx free path

'ath10k_htt_tx_free_cont_txbuf' and 'ath10k_htt_tx_free_cont_frag_desc'
have NULL pointer checks to avoid crash if they are called twice
but this is as of now not sufficient as these pointers are not assigned
to NULL once the contiguous DMA memory allocation is freed, fix this.
Though this may not be hit with the explicity check of state variable
'tx_mem_allocated' check, good to have this addressed as well.

Below BUG_ON is hit when the above scenario is simulated
with kernel debugging enabled

 page:f6d09a00 count:0 mapcount:-127 mapping:  (null)
index:0x0
 flags: 0x40000000()
 page dumped because: VM_BUG_ON_PAGE(page_ref_count(page)
== 0)
 ------------[ cut here ]------------
 kernel BUG at ./include/linux/mm.h:445!
 invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
 EIP is at put_page_testzero.part.88+0xd/0xf
 Call Trace:
  [<c118a2cc>] __free_pages+0x3c/0x40
  [<c118a30e>] free_pages+0x3e/0x50
  [<c10222b4>] dma_generic_free_coherent+0x24/0x30
  [<f8c1d9a8>] ath10k_htt_tx_free_cont_txbuf+0xf8/0x140

  [<f8c1e2a9>] ath10k_htt_tx_destroy+0x29/0xa0

  [<f8c143e0>] ath10k_core_destroy+0x60/0x80 [ath10k_core]
  [<f8acd7e9>] ath10k_pci_remove+0x79/0xa0 [ath10k_pci]
  [<c13ed7a8>] pci_device_remove+0x38/0xb0
  [<c14d3492>] __device_release_driver+0x72/0x100
  [<c14d36b7>] driver_detach+0x97/0xa0
  [<c14d29c0>] bus_remove_driver+0x40/0x80
  [<c14d427a>] driver_unregister+0x2a/0x60
  [<c13ec768>] pci_unregister_driver+0x18/0x70
  [<f8aced4f>] ath10k_pci_exit+0xd/0x2be [ath10k_pci]
  [<c1101e78>] SyS_delete_module+0x158/0x210
  [<c11b34f1>] ? __might_fault+0x41/0xa0
  [<c11b353b>] ? __might_fault+0x8b/0xa0
  [<c1001a4b>] do_fast_syscall_32+0x9b/0x1c0
  [<c178da34>] sysenter_past_esp+0x45/0x74

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>