SCSI Polled io fixes for mac68k with PDMA enabled.

[Nathanial Sloss <nathanialsloss%yahoo.com.au@localhost>]

Hi,

I've found while working with mac68k that devices that require polled io scsi 
transfers would fail with sbc pdma (pseudo dma) would fail when reading and 
writing to the respective device.

I've found that virtual devices for rascsi and the scsi2sd drives would fail 
using pdma.  For virtual disks they would fail always when writing to the 
device, reads were ok.

For virtual ethernet devices they would fail for reading and writing.

To address this I've inctrouduced a flag for sbc.4 PDMA_NO_WRITE which would 
fallback consistently to polled io when writing to the deivice.

Also to check the current scsi transfer control flag XS_CTL_POLL and if set it 
would not use PDMA for that particular transfer.

Please see:

ftp.netbsd.org/pub/NetBSD/misc/nat/sbc_poll_fix.diff

Any objections?

Best regards,

Nat