thermal: ti-soc-thermal: clean up the error handling a bit

We don't need to initialize "ret".  We can move the IS_ERR() checks into
the if condition instead of doing an assignment first.  Also there is a
check for "ret" when we know it is zero so we can remove that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>