ath10k: Fix the size used in a 'dma_free_coherent()' call in an error handling path

Update the size used in 'dma_free_coherent()' in order to match the one
used in the corresponding 'dma_alloc_coherent()'.

Fixes: 18630083

 ("ath10k: fix shadow register implementation for WCN3990")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200802122227.678637-1-christophe.jaillet@wanadoo.fr