When using mailbox to request firmware to enable or disable PCIe bus
master, there is a small probability that mailbox cannot respond.
Change to use rte_pci_read_config() and rte_pci_write_config(), to
avoid this problem.
Fixes: ac6c5e9af5 ("net/ngbe: fix Tx hang on queue disable")
Cc: stable@dpdk.org
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>