staging: ti-soc-thermal: make unexported functions local

Move _ti_bandgap_write_threshold and _ti_bandgap_read_threshold to static
area, as they are local functions.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>