power: supply: twl4030-charger: make twl4030_bci_property_is_writeable static

The function twl4030_bci_property_is_writeable can be made static
as it does not need to be in global scope.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: "H. Nikolaus Schaller" <hns@goldelico.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>