wifi: rtw89: fix a width vs precision bug

The "buf" is skb->data that comes from the firmware.  We want to print
"len" number of bytes.  But there is a missing period so the "len"
variable is used for formatting (width) instead of limiting the output
(precision).

Fixes: cad2bd8a

 ("wifi: rtw89: support firmware log with formatted text")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/0700c7b9-bfd3-4aa6-82bf-5bf3c74644e1@moroto.mountain