serial: msm-geni: handle devm_clk_get() errors

devm_clk_get() returns an ERR_PTR on failure, not null. Fix the check to
avoid the board crashing when the clock isn't available.

Additionally, add the missing error handling for this function.

Fixes: 324df15a ("serial: qcom: add support for GENI serial driver")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>