clk: tegra: fix __clk_lookup() return value checks

In case of error, the function __clk_lookup() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should
be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>