thermal/drivers/tsens: Don't hardcode sensor slope

Function compute_intercept_slope hardcode the sensor slope to
SLOPE_DEFAULT. Change this and use the default value only if a slope is
not defined. This is needed for tsens VER_0 that has a hardcoded slope
table.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Thara Gopinath <thara.gopinath@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210420183343.2272-2-ansuelsmth@gmail.com