USB: xhci: Refactor test for vendor-specific completion codes.

All commands that can be issued to the xHCI hardware can come back with
vendor-specific "informational" completion codes.  These are to be treated
like a successful completion code.  Refactor out the code to test for the
range of these codes and print debugging messages.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>