thermal: tegra_soctherm: fix sign bit of temperature

The sign bit of temperature readback is bit 0, not bit 1.
Change to BIT(0) to fix it.

Signed-off-by: Wei Ni <wni@nvidia.com>
Reviewed-by: Matt Longnecker <mlongnecker@nvidia.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>