r8169: Preserve other Device Control bits when setting NOSNOOP_EN

Previously, when we turned on the "Enable No Snoop Bit," we cleared all
the other Device Control bits, including error reporting enables,
Max_Payload_Size, Max_Read_Request_Size, etc.  This patch preserves
all the other bits.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>