[SCSI] megaraid_{mm,mbox}: 64-bit DMA capability checker

This patch contains
- a fix for 64-bit DMA capability check in megaraid_{mm,mbox} driver.
- includes changes (going back to 32-bit DMA mask if 64-bit DMA mask
failes) suggested by James with previous patch.
- addition of SATA 150-4/6 as commented by Vasily Averin.

With patch, the driver access PCIconfiguration space with dedicated
offset to read a signature. If the signature read, it means that the
controller has capability to handle 64-bit DMA.
Without this patch, the driver used to blindly claim 64-bit DMA
capability.
The issue has been reported by Vasily Averin [vvs@sw.ru].
Thank you Vasily for the reporting.

Signed-Off By: Seokmann Ju <seokmann.ju@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>