PCI: add pci_try_set_mwi

As suggested by Andrew, add pci_try_set_mwi(), which does not require
return-value checking.

- add pci_try_set_mwi() without __must_check
- make it return 0 on success, errno if the "try" failed or error
- review callers

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/gadget/net2280.c b/drivers/usb/gadget/net2280.c
index d975ecf..00fda33 100644
--- a/drivers/usb/gadget/net2280.c
+++ b/drivers/usb/gadget/net2280.c
@@ -2964,7 +2964,7 @@
 			, &dev->pci->pcimstctl);
 	/* erratum 0115 shouldn't appear: Linux inits PCI_LATENCY_TIMER */
 	pci_set_master (pdev);
-	pci_set_mwi (pdev);
+	pci_try_set_mwi (pdev);
 
 	/* ... also flushes any posted pci writes */
 	dev->chiprev = get_idx_reg (dev->regs, REG_CHIPREV) & 0xffff;