[SCSI] sym53c8xx: fix resid calculation

This patch fixes the calculation of the data transfer residual for the
case of a command that is supposed to transfer an odd number of bytes on
a wide bus but transfers nothing instead.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Cc: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>