lan78xx: Fix Smatch Warnings

lan78xx.c:2282 tx_complete() warn: variable dereferenced before check 'skb' (see line 2249)
lan78xx.c:2885 lan78xx_bh() info: ignoring unreachable code.
lan78xx.c:3159 lan78xx_probe() info: ignoring unreachable code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>