sound: Make DSP_FIXUP_ERROR() regular code

No reason to have this as a macro.

While here, change the second case to an "else if" as there is no reason
to check it if the open flags are invalid.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D45776