rtw88: debug: Fix uninitialized memory in debugfs code

This code does not ensure that the whole buffer is initialized and none
of the callers check for errors so potentially none of the buffer is
initialized.  Add a memset to eliminate this bug.

Fixes: e3037485

 ("rtw88: new Realtek 802.11ac driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/X8ilOfVz3pf0T5ec@mwanda