[SCSI] tmscsim: set max_sectors

AM53C974A's Start Transfer Counter register has 24 bits, thus
maximum transfer length is 16MiB. But the maximum I can test
is 8MiB, so use that until somebody tests 16MiB.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>