power: supply: generic-adc-battery: simplify read_channel logic

Drop mostly useless gab_prop_to_chan() function by directly
supplying the correct enum value to read_channel().

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>