xhci: Fix missing break in xhci_evaluate_context_result.

Coverity complains that xhci_evaluate_context_result() is missing a
break statement after the COMP_EBADSLT switch case.  It's not a big
deal, since we wanted to return the same error code as the case
statement below it does.  The end result would be one that a Slot
Disabled error completion code would also print the warning message
associated with a Context State error code.  No other bad behavior would
result.

It's not worth backporting to stable kernels, since it only fixes an
issue with too much debugging.

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