greybus: power_supply: change property values to integer

To align with power supply core values type (integer) move the val and
previous_val to integer also.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>