PCI: read-modify-write the pcie device control register when initiating pcie flr

The pcie_flr routine writes the device control register with the FLR bit
set clearing all other fields for the FLR duration. Among other fields,
the Max_Payload_Size is also cleared which can cause errors if there are
transactions lurking in the HW pipeline. The patch replaces the blank
write with read-modify-write of the control register keeping the other
fields intact.

Signed-off-by: Shmulik Ravid <shmulikr@broadcom.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>