pci/dmar: correct off-by-one error in dmar_fault()

DMAR faults are recorded into a ring of "fault recording registers".
fault_index is a 0-based index into the ring. The code allows the
0-based fault_index to be equal to the total number of fault registers
available from the cap_num_fault_regs() macro, which causes access
beyond the last available register.

Signed-off-by Troy Heber <troy.heber@hp.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>