[SCSI] ps3rom: fix wrong resid calculation bug

sg driver rounds up the length in struct scatterlist to be a multiple
of 512 in some conditions. So LLDs can't use the data length in a sg
list to calculate residual. Instead, the length in struct scsi_cmnd
should be used.

[Geert: the variable buflen already contains scsi_bufflen(cmd)]

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>