net: usb: asix_devices: fix missing return code check on call to asix_write_medium_mode

The call to asix_write_medium_mode is not updating the return code ret
and yet ret is being checked for an error. Fix this by assigning ret to
the return code from the call asix_write_medium_mode.

Detected by CoverityScan, CID#1357148 ("Logically Dead Code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>