mfd: Fix building without CONFIG_OF

Using the of_fwnode_handle() means that local 'node' variables are unused
whenever CONFIG_OF is disabled for compile testing:

drivers/mfd/88pm860x-core.c: In function 'device_irq_init':
drivers/mfd/88pm860x-core.c:576:29: error: unused variable 'node' [-Werror=unused-variable]
  576 |         struct device_node *node = i2c->dev.of_node;
      |                             ^~~~
drivers/mfd/max8925-core.c: In function 'max8925_irq_init':
drivers/mfd/max8925-core.c:659:29: error: unused variable 'node' [-Werror=unused-variable]
  659 |         struct device_node *node = chip->dev->of_node;
      |                             ^~~~
drivers/mfd/twl4030-irq.c: In function 'twl4030_init_irq':
drivers/mfd/twl4030-irq.c:679:46: error: unused variable 'node' [-Werror=unused-variable]
  679 |         struct                  device_node *node = dev->of_node;
      |                                              ^~~~

Replace these with the corresponding dev_fwnode() lookups that
keep the code simpler in addition to avoiding the warnings.

Fixes: e3d44f11da04 ("mfd: Switch to irq_domain_create_*()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250520154106.2019525-1-arnd@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>