ath10k: Fix an error code in ath10k_add_interface()

When the code execute this if statement, the value of ret is 0.
However, we can see from the ath10k_warn() log that the value of
ret should be -EINVAL.

Clean up smatch warning:

drivers/net/wireless/ath/ath10k/mac.c:5596 ath10k_add_interface() warn:
missing error code 'ret'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: ccec9038 ("ath10k: enable raw encap mode and software crypto engine")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1621939577-62218-1-git-send-email-yang.lee@linux.alibaba.com