nvmet: fix a width vs precision bug in nvmet_subsys_attr_serial_show()

This was intended to limit the number of characters printed from
"subsys->serial" to NVMET_SN_MAX_SIZE.  But accidentally the width
specifier was used instead of the precision specifier so it only
affects the alignment and not the number of characters printed.

Fixes: f0406481

 ("nvmet: fixup buffer overrun in nvmet_subsys_attr_serial()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>