bnxt_en: Refactor bnxt_fw_to_ethtool_advertised_spds().

Include the conversion of pause bits and add one extra call layer so
that the same refactored function can be reused to get the link partner
advertisement bits.

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