thunderbolt: Check return value of kmemdup() in icm_handle_event()

kmemdup() may return NULL if there is not enough memory available. Check
this and bail out early in this case. While there move INIT_WORK() to
happen after we have allocated all the memory needed for the event
handling to avoid doing unnecessary work.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>