irqchip: vic: update the base IRQ member correctly

When passing 0 as the irq base the VIC driver will dynamically
allocate a number of consecutive interrupt descriptors at some
available number range. Make sure this number is recorded in
the state container rather than the passed-in zero argument
in this case.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>