hwmon: (mcp3021) Fix broken output scaling

The mcp3021 scaling code is dividing the VDD (full-scale) value in
millivolts by the A2D resolution to obtain the scaling factor. When VDD
is 3300mV (the standard value) and the resolution is 12-bit (4096
divisions), the result is a scale factor of 3300/4096, which is always
one.  Effectively, the raw A2D reading is always being returned because
no scaling is applied.

This patch fixes the issue and simplifies the register-to-volts
calculation, removing the unneeded "output_scale" struct member.

Signed-off-by: Nick Stevens <Nick.Stevens@digi.com>
Cc: stable@vger.kernel.org # v3.10+
[Guenter Roeck: Dropped unnecessary value check]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>