ath11k: fix up some error paths

There are two error paths where "ret" wasn't set.  Also one error path
we set the error code to -EINVAL but we should just preserve the error
code from ath11k_hal_srng_get_entrysize().  That function only returns
-EINVAL so this doesn't change anything.

I removed the "ret = 0;" initializers so that hopefully GCC will be able
to detect these sorts of bugs in the future.

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>