serial: qcom-geni: move clock-rate logic to separate function

serial: qcom-geni: move clock-rate logic to separate function

Facilitates future modifications within the new function,
leading to better readability and maintainability of the code.

Move the code that handles the actual logic of clock-rate
calculations to a separate function geni_serial_set_rate()
which enhances code readability.

Signed-off-by: Praveen Talari <quic_ptalari@quicinc.com>
Link: https://lore.kernel.org/r/20250721174532.14022-7-quic_ptalari@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>