bnxt_en: Enhance hwmon temperature reporting

Driver currently does hwmon device register and unregister
in open and close() respectively. As a result, user will not
be able to query hwmon temperature when interface is in
ifdown state.

Enhance it by moving the hwmon register/unregister to the
probe/remove functions.

Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>