ath11k: Adding proper validation before accessing tx_stats

Before dumping tx_stats proper validation was not been taken care of.
Due to which we were encountering null pointer dereference(kernel panic).
This scenario will arise when a station is getting disconnected and
we are changing the STA state by ath11k_mac_op_sta_state and assigning
tx_stats as NULL and after this the mac80211 will destroy the
debugfs entry from where we are trying to read the stats.

If anyone tries to dump tx_stats for that STA in between setting
tx_stats to NULL and debugfs file removal without checking the NULL
value it will run into a NULL pointer exception.

Proceeding with the analysis of "ARM Kernel Panic".
The APSS crash happened due to OOPS on CPU 3.
Crash Signature : Unable to handle kernel NULL pointer dereference at
virtual address 00000360
During the crash,
PC points to "ath11k_debug_htt_stats_init+0x16ac/0x1acc [ath11k]"
LR points to "ath11k_debug_htt_stats_init+0x1688/0x1acc [ath11k]".
The Backtrace obtained is as follows:
[<ffffffbffcfd8590>] ath11k_debug_htt_stats_init+0x16ac/0x1acc [ath11k]
[<ffffffc000156320>] do_loop_readv_writev+0x60/0xa4
[<ffffffc000156a5c>] do_readv_writev+0xd8/0x19c
[<ffffffc000156b54>] vfs_readv+0x34/0x48
[<ffffffc00017d6f4>] default_file_splice_read+0x1a8/0x2e4
[<ffffffc00017c56c>] do_splice_to+0x78/0x98
[<ffffffc00017c63c>] splice_direct_to_actor+0xb0/0x1a4
[<ffffffc00017c7b4>] do_splice_direct+0x84/0xa8
[<ffffffc000156f40>] do_sendfile+0x160/0x2a4
[<ffffffc000157980>] SyS_sendfile64+0xb4/0xc8

Signed-off-by: Pravas Kumar Panda <kumarpan@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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