Only read 32 bits for the BLK status register in read_blk_stat().

The format and size of this register is defined in the
"NVDIMM Driver Writer's guide":

http://pmem.io/documents/NVDIMM_Driver_Writers_Guide.pdfSigned-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reported-by: Nicholas Moulin <nicholas.w.moulin@linux.intel.com>
Tested-by: Nicholas Moulin <nicholas.w.moulin@linux.intel.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>