mt76: mt7921: fix a precision vs width bug in printk

Precision %.*s was intended instead of width %*s.  The original code
is potentially an information leak.

Fixes: c7cc5ec5 ("mt76: mt7921: rework mt7921_mcu_debug_msg_event routine")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>