bnxt_en: report signal mode in link up messages

Firmware reports link signalling mode for certain speeds. In these
cases, print the signalling modes in kernel log link up messages.

Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>