clk: qcom: Return error pointers for unimplemented clocks

Not all clocks are implemented but client drivers can still
request them. Currently we will return a NULL pointer to them if
the clock isn't implemented in software but NULL pointers are
valid clock pointers. Return an error pointer so that driver's
don't proceed without a clock they may actually need.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>