irqchip/crossbar: Fix incorrect type of register size

The 'size' variable is unsigned according to the dt-bindings.
As this variable is used as integer in other places, create a new variable
that allows to fix the following sparse issue (-Wtypesign):

  drivers/irqchip/irq-crossbar.c:279:52: warning: incorrect type in argument 3 (different signedness)
  drivers/irqchip/irq-crossbar.c:279:52:    expected unsigned int [usertype] *out_value
  drivers/irqchip/irq-crossbar.c:279:52:    got int *<noident>

Signed-off-by: Franck Demathieu <fdemathieu@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>