serial: msm_geni_serial: Enable interrupt post handler registration

UART resume is happening earlier during the probe and driver don't
expect any transfer that time. Hence enable interrupt after the
port open where handler gets registered.

Change-Id: I3fe63be5c4fb244cd8d37831248374d97690bb72
Signed-off-by: Mitul Golani <mgolani@codeaurora.org>