bnxt_en: support lane configuration via ethtool

Recent kernels support changing the number of link lanes via ethtool.
This is useful for determining the appropriate signal mode to use when
a given link speed can be achieved using different lane configurations.

Accept the ethtool lanes parameter when configuring forced speed.  If
there is no lanes parameter, select a default.

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>