If tegra_dfll_unregister() fails then "soc" is an error pointer.  We
should just return instead of dereferencing it.

Fixes: 1752c9ee ("clk: tegra: dfll: Fix drvdata overwriting issue")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>