Deprecate the 'devclass' argument from *DRIVER_MODULE() macros.

This argument is useless for the vast majority of drivers. For now,
use VA_ARGS wrapper macros so that that the *DRIVER_MODULE()
macros accept both the old version (with a devclass) and the new
version (which omits the argument and stores NULL in the
driver_module_data structure). This provides an API compatiblity
shim that can be merged to older stable branches.

Once all drivers relevant to 14.0 (both in and out of tree) have been
updated, the API compat shims can be dropped.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D34963