mfd: 88pm860x: Remove unused extern declarations

commit 260a127b ("mfd: 88pm860x-i2c: Purge unused functions")
left behind this.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20230728132841.10648-1-yuehaibing@huawei.com


Signed-off-by: Lee Jones <lee@kernel.org>