net: lwm2m: Fix binary to float32_value_t conversion

The helper function to conver 32-bit binary float value to
float32_value_t incorrectly identified the "hidden" bit, resulting in
invalid conversion when TLV encoding was used to write a resource
(the output value was divided by 2).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>