sh_eth: exit probe with unknown register layout

Exit the driver's probe() method when the register layout is unknown as the
driver would cause kernel oops in this case anyway.

While at it, move the corresponding error message printout and convert it from
pr_err() to dev_err().

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>