serial: msm_geni_serial: Check if workqueue allocation fails

Pointer wakeup_irq_wq can be NULL if workqueue
allocation fails and may be dereferenced later.

To avoid this returned error if workqueue
allocation fails.

Change-Id: Ic70c862e75c2594171c6a4f165ea87f21524cd40
Signed-off-by: Visweswara Tanuku <quic_vtanuku@quicinc.com>
Signed-off-by: Panicker Harish <quic_pharish@quicinc.com>