bnxt: don't set media status if link is down

When the link is down don't set flags other than IFM_AVALID & IFM_ETHER
This avoids media: Ethernet autoselect (Unknown <full-duplex>) on ifconfig

Reviewed by: zlei, pouria
Approved by: glebius (mentor)
MFC after: 1 week
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D54573

(cherry picked from commit 69de2b4e96794793f04d1b323d354fcde3c480f6)