[PATCH v7 6/8] serial: qcom-geni: move clock-rate logic to separate function

[Praveen Talari <quic_ptalari@xxxxxxxxxxx>]

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@xxxxxxxxxxx>
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v6 -> v7