bnxt: add support for Flow control setting using sysctl

Created sysctl node dev.bnxt.0.fc with following options.

A. dev.bnxt.0.fc.autoneg
B. dev.bnxt.0.fc.rx
C. dev.bnxt.0.fc.tx

Description:-
dev.bnxt.0.fc: flow ctrl
dev.bnxt.0.fc.autoneg: Enable or Disable Autoneg Flow Ctrl: 0 / 1
dev.bnxt.0.fc.rx: Enable or Disable Rx Flow Ctrl: 0 / 1
dev.bnxt.0.fc.tx: Enable or Disable Tx Flow Ctrl: 0 / 1

Submitted by: Bhargava Chenna Marreddy <bhargava.marreddy@broadcom.com>
Reviewed by: shurd, sbruno
Approved by: sbruno (mentor)
Sponsored by: Broadcom Limited
Differential Revision: https://reviews.freebsd.org/D12599