drivers/net/irda/mcs7780.c: fix error return code

The function mcs_probe() return 0 for success and negative value
for most of its internal tests failures. There is one exception
that is error case going to error2:. For 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.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>