serial: msm_geni_serial: Ensure to set clock freq correctly

This patch enables UART clock to run at half the rate and make
sure write to register gets executed with read back of the same
regiter.

Change-Id: Ic4087f6e246b112769335e883a35d8eb00412907
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>