[SCSI] ps3rom: Simplify fill_from_dev_buffer()

As we no longer need to calculate the data length of the whole scatterlist,
we can abort the loop earlier and coalesce req_len and act_len into one
variable, making fill_from_dev_buffer() more similar to fetch_to_dev_buffer().

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>