mpt2sas: Do not check DIF for unwritten blocks

mpt2sas: Do not check DIF for unwritten blocks

Blocks that have never been written contain all ones in the protection
information. When reading, a target device will disable checking for any
block whose application tag contains 0xFFFF.

Some MPT adapters, however, enable checking for all blocks by
default. This causes reads to previously unwritten sectors to fail.

Tweak the relevant bit in the MPT configuration page that causes the HBA
to disable PI checking when a received app tag contains 0xFFFF.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>