i2c: mlxbf: depend on ACPI; clean away ifdeffage

This fixes maybe_unused warnings/errors.

According to a comment during device tree removal, only ACPI is supported,
thus let's actually require it.

Fixes: be18c5ed

 ("i2c: mlxbf: remove device tree support")
Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Wolfram Sang <wsa@kernel.org>