bnxt_en: Skip disabling autoneg before PHY loopback when appropriate.

New firmware allows PHY loopback to be set without disabling autoneg
first.  Check this capability and skip disabling autoneg when
it is supported by firmware.  Using this scheme, loopback will
always work even if the PHY only supports autoneg.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>