drivers: smbus: use correct device initialization priority

The smbus driver initialization priority was previously set to
`KERNEL_INIT_PRIORITY_DEFAULT` (which is 40). However, the default init
priority of devices is typically `KERNEL_INIT_PRIORITY_DEVICE` (which is
50).

Since the stm32 smbus driver uses a reference to the underlying i2c
device, and since this driver was not being built in CI, this led to the
discovery that the smbus driver was initialized too early.

```shell
ERROR: Device initialization priority validation failed, the sequence of \
initialization calls does not match the devicetree dependencies.
ERROR: /smbus1 <smbus_stm32_init> is initialized before its dependency \
/soc/i2c@40005400 <i2c_stm32_init> (POST_KERNEL+1 < POST_KERNEL+4)
```

By setting the initialization priority to `KERNEL_INIT_PRIORITY_DEVICE`,
both smbus1 and i2c1 have their priorities evaluated in the same group,
and it becomes possible to determine relative priorities via phandle
dependency.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

Author: cfriedt

drivers/smbus/Kconfig

@@ -24,7 +24,7 @@ config SMBUS_STATS
 
 config SMBUS_INIT_PRIORITY
 	int "Init priority"
-	default KERNEL_INIT_PRIORITY_DEFAULT
+	default KERNEL_INIT_PRIORITY_DEVICE
 	help
 	  SMBus device driver initialization priority.