drivers/video/matrox/matroxfb_maven.c: fix unsigned return type

The function has an unsigned return type, but returns a negative constant
to indicate an error condition.  The result of calling the function is
only tested by comparison to 0, and thus unsigned is not needed and can be
dropped from the return type.

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>
Cc: Petr Vandrovec <vandrove@vc.cvut.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>