afc: fix wrong enum value

Value of sbservices_error_t type is compared with INSTPROXY_E_SUCCESS
from instproxy_error_t, SBSERVICES_E_SUCCESS should be used instead.
However it isn't serious, because both constants have same value...

It fixes following warning:
gvfsbackendafc.c:262:21: warning: comparison between ‘sbservices_error_t’ and ‘enum <anonymous>’ [-Wenum-compare]
if (G_LIKELY(cond == INSTPROXY_E_SUCCESS))