tty: serial: qcom_geni_serial: Use min3 to find minimum of 3 values

Use min3 helper to calculate the minimum value of 3 variables.

Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>