wifi: wcn36xx: Slightly optimize PREPARE_HAL_BUF()

In most (likely all) cases, INIT_HAL_MSG() is called before
PREPARE_HAL_BUF().
In such cases calling memset() is useless because:
   msg_body.header.len = sizeof(msg_body)

So, instead of writing twice the memory, we just have a sanity check to
make sure that some potential trailing memory is zeroed.
It even gives the opportunity to see that by itself and optimize it away.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/7d8ab7fee45222cdbaf80c507525f2d3941587c1.1675371372.git.christophe.jaillet@wanadoo.fr