mfd: max77620: Do not allocate IRQs upfront

regmap_add_irq_chip() will try to allocate all of the IRQ descriptors
upfront if passed a non-zero irq_base parameter. However, the intention
is to allocate IRQ descriptors on an as-needed basis if possible. Pass 0
instead of -1 to fix that use-case.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>