ath11k: fix missing free of skb on error return path

The error handling when the call to ath11k_hal_srng_get_entrysize fails
leaks skb, fix this by returning via the err_free return path that will
ensure the skb is free'd.

Addresses-Coverity: ("Resource leak")
Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>