xhci 1.0: Only interrupt on short packet for IN EPs.

It doesn't make sense to set the interrupt on short packet (TRB_ISP) flag
for TRBs queued to endpoints that only receive packets from the host
controller (i.e. OUT endpoints).  Packets can only be short when they are
sent from a USB device.  Plus, the xHCI 1.0 specification forbids setting
the flag for anything but IN endpoints.

While we're at it, remove some of my snide remarks about the inefficiency
of event data TRBs.

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