mt76: mt7915: fix a precision vs width bug in printk

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

Fixes: 665b2c78 ("mt76: mt7915: 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>