serial: msm_geni_serial: No need to stop_tx/rx on UART shutdown

Framework driver already stopping rx/tx before calling port shutdown
api to ensure everything is stopped before disabling port. Hence
removing stop_rx/tx from port shutdown function.

Change-Id: I53891f37dae146b7960c123c00cac6cdfd81289d
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>