mt76: mt7615: fix a precision vs width bug in printk

Precision "%.*s" was intended instead of width "%*s".  The original code
will print garbage from beyond the end of the skb->data.

Fixes: d76d6c3b ("mt76: mt7615: limit firmware log message printk to buffer length")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>