xhci: Remove BUG in xhci_setup_addressable_virt_dev

We may have more speed types in the future, so fail gracefully, rather
than causing the kernel to panic.

BUG() was called if the device speed was unknown when setting max packet
size.  Set the max packet size at the same time as the slot speed and
get rid of one switch statement with BUG() option completely.

[Note: Sarah merged a patch that she wrote that touched the
xhci_setup_addressable_virt_dev function with this patch from Mathias
for clarity.]

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