[libata] pata_bf54x: Support sg list in bmdma transfer.

BF54x on-chip ATAPI controller allows maximum 0x1fffe bytes to be transfered
in one ATAPI transfer. So, set the max sg_tablesize to 4.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>