This patch addresses two issues with to_dev_zio:

1. the argument of the macro is never used (_dev).

2. as to_dev_zio is taking a struct fmcadc_dev member as an argument
supposed to be a gid, the compiler spits a warning. A (void *) cast
is a quick and dirty fix (actually, dev should be casted everywhere
to struct fmcadc_gid *, but anyway).