thermal: rcar_gen3_thermal: remove unneeded mutex

There is no point in protecting a register read with a lock. This is
most likely a leftover from when the driver was reworked before being
submitted for upstream.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>