msm_serial_hs: fix error handling path of wakeup irq

When request_threaded_irq is called, the wakeup_source
is not yet inited. In case where the request_threaded_irq fails,
the failure path should not trash the wakeup_source.

Change-Id: Ib840c32d545265ecfbc4d7d95baa40e230388549
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>