bnxt_en: support hwrm_func_drv_unrgtr command

During remove_one, the driver should issue hwrm_func_drv_unrgtr
command to inform firmware that this function has been unloaded.
This is to let firmware keep track of driver present/absent state
when driver is gracefully unloaded. A keep alive timer is needed
later to keep track of driver state during abnormal shutdown.

Signed-off-by: Jeffrey Huang <huangjw@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>