power: supply: generic-adc-battery: style fixes

This does the following code-style changes:

 * fix inconsistent indentation in 'struct gab'
 * remove unused cable_plugged from 'struct gab'
 * remove pointless temporary is_plugged variable
 * add gab_ prefix to read_channel

No functionality changes are intended.

Signed-off-by: Sebastian Reichel <sre@kernel.org>