libsa/rarp.c: Change casted type and printf format

Change the casted type and printf format string to avoid compilation
failure when RARP_DEBUG macro is defined.

Reviewed by: imp
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D36738

(cherry picked from commit ee0d06faa0a5b0e551cac4cfcad68062cb3257c7)