[SCSI] qla2xxx: Use dma_get_required_mask() in determining the 'ideal' DMA mask.

In order to efficiently utilise the ISP's IOCB
request-queue, use the dma_get_required_mask() function to
determine the use of command-type 2 or 3 IOCBs when queueing
SCSI commands.  This applies to ISP2[123]xx chips only, as
the ISP24xx uses command-type 7 IOCBs which use 64bit DSDs.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>