xhci: rename endpoint related trb variables

Renaming variables, no functional changes

Event trbs on the event ring cointain a pointer to the transfer trb
on a endpoint ring that caused the event.

The naming has been very confusing as the event trb was called "event"
and the endpoint transfer trb was called "event_trb".

rename all variables that are realated to the endpoint transfer trb
from event_* to ep_*

No functional changes

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