bnxt_en: Log unknown link speed appropriately.

bnxt_en: Log unknown link speed appropriately.

If the VF virtual link is set to always enabled, the speed may be
unknown when the physical link is down.  The driver currently logs
the link speed as 4294967295 Mbps which is SPEED_UNKNOWN.  Modify
the link up log message as "speed unknown" which makes more sense.

Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/1602493854-29283-7-git-send-email-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>