regmap: use IS_ERR() to check clk_get() results

clk_get() returns an error pointer, or a valid token to pass back to the
clock API. Hence, the result must be checked with IS_ERR(), not by
comparison against NULL.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>