arm: mach-davinci: check irq2ctlr() result

If irq2ctlr() fails return IRQ_NONE.
Also as it can fail make 'ctlr' signed.

The semantic patch that finds this problem (many false-positive results):
(http://coccinelle.lip6.fr/)

// <smpl>
@ r1 @
identifier f;
@@
int f(...) { ... }

@@
identifier r1.f;
type T;
unsigned T x;
@@

*x = f(...)
 ...
*x > 0

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>