bnxt_en: Added support for priority queues extended stats

Below priority queues extended stats are exposed to sysctl:

tx_bytes_pri{0-7}
rx_bytes_pri{0-7}
tx_packets_pri{0-7}
rx_packets_pri{0-7}

Reviewed by: imp
Approved by: imp
Differential revision: https://reviews.freebsd.org/D45007