[SCSI] ps3rom: use sg buffer copy helper funcitons

Note that if scsi_bufflen(cmd) is not zero, the command always has an
sg list. So this patch doesn't do the error checking in
fill_from_dev_buffer and fetch_to_dev_buffer did.

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