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: I4568ec8fb471cd24eee27bc4cc3bd76e27ea68d8
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>