gg: Add the ability to set gas gauge deadband

The deadband is the range which is considered to be "0 mA".  This might
help the gas gauge drive itself, and prevent it from going into suspend
mode.

Signed-off-by: Sean Cross <xobs@kosagi.com>
cmd-gg: Implement "deadband" function

This allows users to read the current deadband value, or to set a new
deadband value.

Signed-off-by: Sean Cross <xobs@kosagi.com>