thermal: armada: Simplify the check of the validity bit

All Armada SoCs use one bit to declare if the sensor values are valid.
This bit moves across the versions of the IP.

The method until then was to do both a shift and compare with an useless
flag of "0x1". It is clearer and quicker to directly save the value that
must be ANDed instead of the bit position and do a single bitwise AND
operation.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>