xhci: Remove BUG_ON() in xhci_alloc_container_ctx.

It's horrible coding style to panic the kernel when someone passes you
an argument value you didn't expect.  In the future, we may want to add
additional context types, so it's better to gracefully handle additional
context types instead of panicking.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: John Youn <johnyoun@synopsys.com>