drivers: sensor: sensor_shell: fix channel name collision

Channel SENSOR_CHAN_GAUGE_FULL_CHARGE_CAPACITY was returned in place of
SENSOR_CHAN_GAUGE_FULL_AVAIL_CAPACITY, due to their matching names.
Ensuring that the channel names are unique fixes the issue.

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>