net: explicit cast of multicast bit clearing

GCC 8.1 warned:

rte_ether.h:213:13:
warning: conversion from 'int' to 'uint8_t'
{aka 'unsigned char'} may change value [-Wconversion]
  addr[0] &= ~ETHER_GROUP_ADDR;

Fixes: 7ef007291004 ("ethdev: random MAC address")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>