serial: msm-geni: fix a compile time warning from msm_serial_setbrg()

A compiler warns about a missing function prototype, which is valid
and fixed by converting the function into static one, also fix
interleaved local variable declarations and assignments.

Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Fixes: 324df15a ("serial: qcom: add support for GENI serial driver")
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>