cdc_ether: export usbnet_cdc_zte_rx_fixup

Commit bfe9b9d2 ("cdc_ether: Improve ZTE MF823/831/910 handling")
introduces a workaround for certain ZTE modems reporting invalid MAC
addresses over CDC-ECM.
The same issue was present on their RNDIS interface,which was fixed in
commit a5a18bdf ("rndis_host: Set valid random MAC on buggy devices").

However, internal modem of ZTE MF286R router, on its RNDIS interface, also
exhibits a second issue fixed already in CDC-ECM, of the device not
respecting configured random MAC address. In order to share the fixup for
this with rndis_host driver, export the workaround function, which will
be re-used in the following commit in rndis_host.

Cc: Kristian Evensen <kristian.evensen@gmail.com>
Cc: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>