snd: enable building max98357a driver with ACPIGEN=n

sandbox_defconfig builds the max98357a driver. It should be possible to
build the sandbox without ACPI support.

ACPI support in the max98357a driver is only needed when creating an ACPI
table. Fix building with ACPIGEN=n.

Fixes: 54bcca29 ("sound: Add an ACPI driver for Maxim MAX98357ac")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>