mfd: mt6397: Do not call irq_domain_remove if PMIC unsupported

If the PMIC ID is unknown, the current code would call
irq_domain_remove and panic, as pmic->irq_domain is only
initialized by mt6397_irq_init.

Return immediately with an error, if the chip ID is unsupported.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>