[SCSI] Use SPI messages where possible

Now sym2 is using spi_print_msg, we don't need to have our own messages
for IGNORE WIDE RESIDUE and MODIFY DATA POINTER, so provide the option
of passing NULL for the label.

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