[SCSI] aacraid: use lower snprintf() limit

This is just a cleanup, to silence static checker warnings.  It
doesn't change how the code works.

buf[] can either be BUF_SIZE if this is called from sysfs, or it can
be 16 if it's called from aac_get_adapter_info() via
aac_get_serial_number().  We use the smaller limit here.

sizeof(dev->supplement_adapter_info.MfgPcbaSerialNo) is 12 so there
is actually no chance of hitting either limit.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Achim Leubner <Achim_Leubner@pmc-sierra.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>