msm_serial: fix clock rate on DMA-based uarts

The driver explicitly requests a clock rate for the UART, but it is
off by a factor of four from the dividers that it programs into the
UART.  Fix this by setting the rate to 1/4 of the current value.

Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>