serial: msm: add lock annotation to msm_set_baud_rate()

msm_set_baud_rate() releases and re-acquires the port->lock, thus add
lock annotation for Sparse static code checks.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230109152212.343476-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>