drivers/net/ethernet/natsemi/natsemi.c: fix error return code

The function natsemi_probe1() return 0 for success and negative value
for most of its internal tests failures. There is one exception
that is error case going to err_create_file:. Fore this error case the
function abort its success execution path, but returns non negative value,
making it difficult for a caller function to notice the error.

This patch fixes the error case that do not return negative value.

This was found by Coccinelle, but the code change was made by hand.
This patch is not robot generated.

A simplified version of the semantic match that finds this problem is
as follows: (http://coccinelle.lip6.fr/

)

// <smpl>
(
if@p1 (\(ret < 0\|ret != 0\))
 { ... return ret; }
|
ret@p1 = 0
)
... when != ret = e1
    when != &ret
*if(...)
{
  ... when != ret = e2
      when forall
 return ret;
}
// </smpl>

Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>