clk: qcom: common: Check for rclks before access

The critical devices or critical clocks where the clocks of the device
node requires to be kept enabled should be checked for valid rclk before
update. Thus check for NULL before accessing the rclk.

Change-Id: Ie0cf1486f8cc422de8fce0241bd6e81fd9d7ca2b
Signed-off-by: Taniya Das <tdas@codeaurora.org>