ath10k: add missing error handling

In function ath10k_sdio_mbox_rx_alloc() [sdio.c],
ath10k_sdio_mbox_alloc_rx_pkt() is called without handling the error cases.
This will make the driver think the allocation for skb is successful and
try to access the skb. If we enable failslab, system will easily crash with
NULL pointer dereferencing.

Call trace of CONFIG_FAILSLAB:
ath10k_sdio_irq_handler+0x570/0xa88 [ath10k_sdio]
process_sdio_pending_irqs+0x4c/0x174
sdio_run_irqs+0x3c/0x64
sdio_irq_work+0x1c/0x28

Fixes: d96db25d

 ("ath10k: add initial SDIO support")
Signed-off-by: Claire Chang <tientzu@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>