mailbox: zynqmp-ipi: drop irq_to_desc() call

irq_to_desc() is not exported to loadable modules, so this driver now
fails to link in some configurations:

ERROR: modpost: "irq_to_desc" [drivers/mailbox/zynqmp-ipi-mailbox.ko] undefined!

I can't see a purpose for this call, since the return value is unused
and probably left over from some code refactoring.

Address the link failure by just removing the line.

Fixes: 6ffb1635341b ("mailbox: zynqmp: handle SGI for shared IPI")
Signed-off-by: Arnd Bergmann <[e-mail]>
Tested-by: Tanmay Shah <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/zynqmp-ipi-mailbox.c

···
 		return ret;
 	}
 
-	irq_to_desc(pdata->virq_sgi);
 	irq_set_status_flags(pdata->virq_sgi, IRQ_PER_CPU);
 
 	/* Setup function for the CPU hot-plug cases */