net: lan78xx: fix rx handling before first packet is send

As long the bh tasklet isn't scheduled once, no packet from the rx path
will be handled. Since the tx path also schedule the same tasklet
this situation only persits until the first packet transmission.
So fix this issue by scheduling the tasklet during ndo_open like in usbnet.

Link: https://github.com/raspberrypi/linux/issues/2617


Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet")
Suggested-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>