i2c: i2c-smbus: Use threaded irq for smbalert

Prior to this commit the smbalert_irq was handling in the hard irq
context. This change switch to using a thread irq which avoids the need
for the work thread. Using threaded irq also removes the need for the
edge_triggered flag as the enabling / disabling of the hard irq for level
triggered interrupts will be handled by the irq core.

Without this change have an irq connected to something like an i2c gpio
resulted in a null ptr deferences. Specifically handle_nested_irq calls
the threaded irq handler.

There are currently 3 in tree drivers affected by this change.

i2c-parport driver calls i2c_handle_smbus_alert in a hard irq context.
This driver use edge trigger interrupts which skip the enable / disable
calls. But it still need to handle the smbus transaction on a thread. So
the work thread is kept for this driver.

i2c-parport-light & i2c-thunderx-pcidrv provide the irq number in the
setup which will result in the thread irq being used.

i2c-parport-light is edge trigger so the enable / disable call was
skipped as well.

i2c-thunderx-pcidrv is getting the edge / level trigger setting from of
data and was setting the flag as required. However the irq core should
handle this automatically.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>