ath11k: Fix an error handling path

If 'kzalloc' fails, we must return an error code.

While at it, remove a useless initialization of 'err' which could hide the
issue.

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201122173943.1366167-1-christophe.jaillet@wanadoo.fr