thermal/drivers/ti-soc-thermal: Avoid dereferencing ERR_PTR

On error the function ti_bandgap_get_sensor_data() returns the error
code in ERR_PTR() but we only checked if the return value is NULL or
not. And, so we can dereference an error code inside ERR_PTR.
While at it, convert a check to IS_ERR_OR_NULL.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200424161944.6044-1-sudipm.mukherjee@gmail.com