rtas_errd: fixed truncation of drc_index in drmgr arg list

The drc_index value was being truncated. The snprintf writes up to
"size" bytes including the \0 terminator. The 32bit hex value is
requires 10 bytes plus a byte for the \0.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>