xhci: trace debug messages related to driver initialization and unload

This patch defines a new trace event, which is called xhci_dbg_init
and belongs to the event class xhci_log_msg, and adds tracepoints that
trace the debug statements in the functions used to start and stop the
xhci-hcd driver.

Also, it removes an unnecessary cast of variable val to unsigned int
in xhci_mem_init(), since val is already declared as unsigned int.

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