Simplify the code dealing with the mbuf chain in dp8390_get().
Simplify the code dealing with the mbuf chain in dp8390_get().

From bad@ & mycroft@ NetBSD

Tested by nick@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars