Remove unused HAVE_PCI_SET_MWI.

Link: https://lore.kernel.org/r/03f20cac-708d-7897-c7c7-cb4e63cfd991@gmail.comSigned-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>