xhci: fix sparse warning in xhci-trace.h

xhci: fix sparse warning in xhci-trace.h

This patch fixes the following sparse warnings:
drivers/usb/host/./xhci-trace.h:116:1: warning: cast to restricted __le32
drivers/usb/host/./xhci-trace.h:116:1: warning: cast to restricted __le32
drivers/usb/host/./xhci-trace.h:116:1: warning: restricted __le32 degrades to
integer
drivers/usb/host/./xhci-trace.h:116:1: warning: restricted __le32 degrades to
integer

by converting the field 'trb' of the trace buffer entry structure from array
with elements of type __le32 to an array with elements of type u8.
Into the trb array are copied the contents of the TRB that generated the event.
The trace-cmd tool with the help of plugin_xhci.py will use this field to
parse the TRB contents in a human readable way.

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