PCI/DPC: Fix control register setting

This driver was OR'ing desired bits from the existing control setting.
That could create an invalid DPC Trigger Enabled configuration if the
platform previously set this to "ERR_FATAL", 01b.  The driver currently
wants to set this to ERR_NONFATAL/ERR_FATAL, 10b, and the logical OR of
this gets 11b, which is reserved.  Fix that by masking off the fields it is
setting.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>