bnxt_en: Add get_eee() and set_eee() ethtool support.

Allow users to get|set EEE parameters.

v2: Added comment for preserving the tx_lpi_timer value in get_eee.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>