ath11k: add missing null check on allocated skb

ath11k: add missing null check on allocated skb

Currently the null check on a newly allocated skb is missing and
this can lead to a null pointer dereference is the allocation fails.
Fix this by adding a null check and returning -ENOMEM.

Addresses-Coverity: ("Dereference null return")
Fixes: 43ed15e1ee01 ("ath11k: put hw to DBS using WMI_PDEV_SET_HW_MODE_CMDID")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201214232417.84556-1-colin.king@canonical.com