qoriq_therm.c: avoid a segfault on the error exit path.

If anything goes wrong during attach() it is handled with a 'goto fail'
which calls sysctl_ctx_free(). But the sysctl context doesn't get
initialized until very late in attach(), so almost any error just results
in a segfault. Move the sysctl_ctx_init() call to the beginning of the
attach() function, so that it is done before any errors can happen that
will lead to freeing the context.