drivers: hwinfo: ensure z_vrfy_hwinfo_get_device_eui64() uses int

The z_vrfy function for this syscall

```
__syscall int hwinfo_get_device_eui64(uint8_t *buffer);
```

in `drivers/hwinfo/hwinfo_handlers.c`, used an `ssize_t` rather than an
`int` which makes it inconsistent with the prototype declared in
`drivers/hwinfo.h`.

Use `int` instead of `ssize_t`.

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

drivers/hwinfo/hwinfo_handlers.c

@@ -15,7 +15,7 @@ ssize_t z_vrfy_hwinfo_get_device_id(uint8_t *buffer, size_t length)
 }
 #include <zephyr/syscalls/hwinfo_get_device_id_mrsh.c>
 
-ssize_t z_vrfy_hwinfo_get_device_eui64(uint8_t *buffer)
+int z_vrfy_hwinfo_get_device_eui64(uint8_t *buffer)
 {
 	K_OOPS(K_SYSCALL_MEMORY_WRITE(buffer, 8));