xhci: clean up event ring checks from inc_enq()

Remove the event ring related checks in inc_enq()

Host hardware is the producer of events on the event ring,
driver will not queue anything, or call inc_enq() for the
event ring.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>