tty: serial: msm: fix definition of msm_stop_dma

The msm_stop_dma() is not exported from the driver, so make
it static to stop the following warning:

drivers/tty/serial/msm_serial.c:84:6: warning: symbol 'msm_stop_dma' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Andy Gross <andy.gross@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>