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: 324df15a29 ("serial: qcom: add support for GENI serial driver")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>