USB: xhci: simplify logic of skipping missed isoc TDs

The logic of the handling Missed Service Error Events was pretty
confusing as we were checking the same condition several times.
In addition, it caused compiler warning since the compiler could
not figure out that event_trb is actually unused in case we are
skipping current TD.

Fix that by rearranging "skip" condition checks, and factor out
skip_isoc_td() so that it is called explicitly.

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