EDAC, mpc85xx: Fix PCIe error capture

EDAC, mpc85xx: Fix PCIe error capture

According to the reference manual of MPC8572 and T4240, bit 31 of
PEX_ERR_CAP_STAT is W1C (write 1 to clear).

Add the corresponding write to PEX_ERR_CAP_STAT in order to fix the PCIe
error capture.

Tested on a T4240 processor.

Signed-off-by: Tillmann Heidsieck <theidsieck@leenox.de>
Acked-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20160815190849.29327-1-theidsieck@leenox.de
Signed-off-by: Borislav Petkov <bp@suse.de>