[SCSI] sym2: Mask off opcode from RBC

pm->sg.size is set from the Residual Byte Count register.  However,
the upper byte of the RBC is the opcode of the instruction that was
executing, so we need to mask it off.  This fixes some spurious rejects
of IGNORE WIDE RESIDUE messages.

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