Fix the following clang 4.0.0 warning in ngatm's snmp_atm.c:
contrib/ngatm/snmp_atm/snmp_atm.c:173:6: error: logical not is only
applied to the left hand side of this bitwise operator
[-Werror,-Wlogical-not-parentheses]
if (!ifmr.ifm_status & IFM_AVALID) {
^ ~Obviously, the masking needs to be done before the logical not
operation. Add parentheses to make it so.
MFC after: 3 days