[PowerPC64] pseries-llan: increment packet output counters on error and success

Summary: when using pseries-llan driver, Opkts and Oerrs counters (netstat
-i) are always zero. This patch adds an small error handling to increment
these counters.

Submitted by: alfredo.junior_eldorado.org.br
Differential Revision: https://reviews.freebsd.org/D20009