rtc: mt6397: Don't call irq_dispose_mapping.

The IRQ mapping was changed to not being created in the rtc-mt6397
driver, so the irq_dispose_mapping is no longer needed.
Also the dev_id passed to free_irq should be the same as the last
argument passed to request_threaded_irq.
This prevents a "Trying to free already-free IRQ 274" warning when
unbinding the driver.

Fixes: e695d3a0 ("mfd: mt6397: Create irq mappings in mfd core driver")
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>