[SCSI] sym2: Allow NVRAM settings to limit speed and width

The NVRAM for both Tekram and Symbios boards allows the user to set the
speed and width for individual targets.  I took that code out in March
2004 when we introduced Domain Validation, but it seems there's still
a legitimate need for it in some configurations.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>