power: supply: generic-adc-battery: remove redundant variable pdata

Pointer pdata is assigned but never used, so remove it. Cleans up the
clang warning:

drivers/power/supply/generic-adc-battery.c:211:2: warning: Value
stored to 'pdata' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>