staging: et131x: Fix link detection signalling

Remove redundant code around netif_carrier_XXX calls, and associated adapter->media_state, which prevented the link from being reported as off.
Tested on an ET-1310 device.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>