[libata] pata_rb532_cf: fix signature of the xfer function

Per definition, this function should return the number of bytes
consumed. As the original parameter "buflen" is being decremented inside
the read/write loop, save it in "retlen" at the beginning.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Acked-by: Sergei Shtyltov <sshtylyov@ru.mvista.com>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>