[SCSI] implement parameter limits in the SPI transport class

There's a basic need not to have parameters go under or over certain
values when doing domain validation.  The basic ones are

max_offset, max_width and min_period

This patch makes the transport class take and enforce these three
limits.  Currently they can be set by the user, although they could
obviously be read from the HBA's on-board NVRAM area during
slave_configure (if it has one).

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>