ath11k: fix crash caused by NULL rx_channel

During connect and disconnect stress test, crashed happened
because ar->rx_channel is NULL. Fix it by checking whether
ar->rx_channel is NULL.

Crash stack is as below:
RIP: 0010:ath11k_dp_rx_h_ppdu+0x110/0x230 [ath11k]
[ 5028.808963]  ath11k_dp_rx_wbm_err+0x14a/0x360 [ath11k]
[ 5028.808970]  ath11k_dp_rx_process_wbm_err+0x41c/0x520 [ath11k]
[ 5028.808978]  ath11k_dp_service_srng+0x25e/0x2d0 [ath11k]
[ 5028.808982]  ath11k_pci_ext_grp_napi_poll+0x23/0x80 [ath11k_pci]
[ 5028.808986]  net_rx_action+0x27e/0x400
[ 5028.808990]  __do_softirq+0xfd/0x2bb
[ 5028.808993]  irq_exit+0xa6/0xb0
[ 5028.808995]  do_IRQ+0x56/0xe0
[ 5028.808997]  common_interrupt+0xf/0xf

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201211055613.9310-1-cjhuang@codeaurora.org