mfd: rk808: Fix RK818_IRQ_DISCHG_ILIM initializer

When building with -Woverride-init, we get a warning about an incorrect
initializer:

drivers/mfd/rk808.c:244:8: error: initialized field overwritten [-Werror=override-init]
  [RK818_IRQ_DISCHG_ILIM] = {

This is clearly a mistake, as both RK818_IRQ_DISCHG_ILIM and RK818_IRQ_USB_OV
are defined as '7', but they refer to different register bits. Changing
RK818_IRQ_DISCHG_ILIM to 15 is consistent with how all other 14 interrupts are
handled here, so I'm assuming this is what it should have been.

Fixes: 2eedcbfc ("mfd: rk808: Add RK818 support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>