drivers/net/wireless/iwlwifi/iwl-agn.c: Fix return value from an unsigned function

The function has an unsigned return type, but returns a negative constant
to indicate an error condition.  Another error condition in the same
function is indicated by returning 0, and indeed the only call to the
function checks for 0 to detect errors, so the return of a negative value
it converted to a return of 0.

A sematic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/

)

// <smpl>
@exists@
identifier f;
constant C;
@@

 unsigned f(...)
 { <+...
*  return -C;
 ...+> }
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>