uart: uart_getenv: check for NULL class last, not first

This allows one to specify dt:XXXX when the default class isn't compiled
into the kernel. It's not an error to not have a class until we're done
parsing the spec, so defer checking until then.

Sponsored by: Netflix
Reviewed by: adrian, andrew, markj
Differential Revision: https://reviews.freebsd.org/D47078