[PATCH] libata: implement ATA_FLAG_SETXFER_POLLING and use it in pata_via, take #2

This patch implements ATA_FLAG_SETXFER_POLLING and use in pata_via.
If this flag is set, transfer mode setting performed by polling not by
interrupt.  This should help those controllers which raise interrupt
before the command is actually complete on SETXFER.

Rationale for this approach.

* uses existing facility and relatively simple
* no busy sleep in the interrupt handler
* updating drivers is easy

While at it, kill now unused flag ATA_FLAG_SRST in pata_via.

Signed-off-by: Tejun Heo <htejun@gmail.com>