xhci: Remove warnings about MSI and MSI-X capabilities.

xHCI host controllers may not be capable of MSI, but they should be able
to be used in legacy PCI interrupt mode.  Similarly, some xHCI host
controllers will have MSI support but not MSI-X support.  Lower the
dmesg log level from an error to debug.  The message won't appear unless
CONFIG_USB_XHCI_HCD_DEBUGGING is turned on.

If we need to find out whether the device can support MSI or MSI-X and
it's not being enabled by the driver, it's easy to ask the user to run
lspci.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>