INTERNAL: ath10k: Avoid possible NULL pointer de-reference

Add check to avoid possible NULL pointer de-reference
in function ath10k_htt_tx_32() and ath10k_htt_tx_64().

Change-Id: I5a4e7e8a9f408085329057dfe11a6dd6b25cda8a
Signed-off-by: Dundi Raviteja <dundi@codeaurora.org>