USB: Avoid NULL pointer deref in usb_hcd_alloc_bandwidth.

usb_ifnum_to_if() can return NULL if the USB device does not have a
configuration installed (usb_device->actconfig == NULL), or if we can't
find the interface number in the installed configuration.  Return an
error instead of crashing.

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