ath10k: Correct error handling of dma_map_single()

The return value of dma_map_single() should be checked for errors using
dma_mapping_error() and the skb has been dequeued so it needs to be
freed.

This was found when enabling CONFIG_DMA_API_DEBUG and it warned about the
missing dma_mapping_error() call.

Fixes: 1807da49

 ("ath10k: wmi: add management tx by reference support over wmi")
Reported-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>