thermal/drivers/hisi: Remove bogus const from function return type

With gcc-4.1.2:

    drivers/thermal/hisi_thermal.c: In function ‘hisi_thermal_probe’:
    drivers/thermal/hisi_thermal.c:530: warning: type qualifiers ignored on function return type

Remove the "const" keyword to fix this.

Fixes: a160a465 ("thermal/drivers/hisi: Prepare to add support for other hisi platforms")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>