irqdomain: Clarify checks for bus_token

The code uses if (bus_token) and if (bus_token == DOMAIN_BUS_ANY).

Since bus_token is an enum, the latter is more robust against changes.

Convert all !bus_token checks to explicitely check for DOMAIN_BUS_ANY.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240812193101.1266625-2-andriy.shevchenko@linux.intel.com