drivers: sensor: ams_as5600: Fix calculation of fractional part

commit 98903d48 upstream.

The original calculation has two bugs. One is the calculated value, and the
other is that the value is not in one-millionth parts.

What the original calculation does is compute a scaled position value by
multiplying the raw sensor value (`dev_data->position`) by
`AS5600_FULL_ANGLE`, which represents a full rotation in degrees. It then
subtracts the product of the whole number of pulses (`val->val1`) and
`AS5600_PULSES_PER_REV` from this scaled position value.

    ((int32_t)dev_data->position * AS5600_FULL_ANGLE)
    - (val->val1 * AS5600_PULSES_PER_REV);

What you actually need is to extract the fractional part of the value by
taking the modulo of AS5600_PULSES_PER_REV from the scaled value of the
position.

   (((int32_t)dev_data->position * AS5600_FULL_ANGLE)
   % AS5600_PULSES_PER_REV)

Then convert the value to one-millionth part.

   * (AS5600_MILLION_UNIT / AS5600_PULSES_PER_REV);

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>