bnxt_en: Fix and clarify link_info->advertising.

The advertising field is closely related to the auto_link_speeds field.
The former is the user setting while the latter is the firmware setting.
Both should be u16.  We should use the advertising field in
bnxt_get_link_ksettings because the auto_link_speeds field may not
be updated with the latest from the firmware yet.

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