power: supply: wm97xx_battery: remove redundant 2nd null check on pdata

pdata is being null checked twice, the 2nd check is redundant code
and can be removed.

Fixes CoverityScan CID 1392340 "Logically dead code"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>