mfd: ab8500: Make ab8500_irq_get_virq() static

MFD core now takes care of  HWIRQ <-> VIRQ mapping, so the helper
ab8500_irq_get_virq() is no longer used by ab8500 subordinate devices
to obtain a Linux wide Virtual IRQ. The AB8500 IRQ controller still
uses it internally though, so we'll just hide it from the rest of the
world by making it static instead.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>