ethtool: thunder_bgx: use ethtool.h constants for speed and duplex

Use constants provided by ethtool.h for speed and duplex values
instead of raw integer constants to increase code readability.

thunder_bgx already uses SPEED_UNKNOWN and DUPLEX_UNKNOWN constants,
also provided by ethtool.h.

Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>