bnxt_en: Handle firmware reset.

Add the bnxt_fw_reset() main function to handle firmware reset.  This
is triggered by firmware to initiate an orderly reset, for example
when a non-fatal exception condition has been detected.  bnxt_fw_reset()
will first wait for all VFs to shutdown and then start the
bnxt_fw_reset_task() work queue to go through the sequence of reset,
re-probe, and re-initialization.

The next patch will add the devlink reporter to start the sequence and
call bnxt_fw_reset().

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>