powerpc/44x/uic: Consolidate chained IRQ handler install/remove

powerpc/44x/uic: Consolidate chained IRQ handler install/remove

The driver currently sets the handler data and the chained handler in
two separate steps. This creates a theoretical race window where an
interrupt could fire after the handler is set but before the data is
assigned, leading to a NULL pointer dereference.

Replace the two calls with irq_set_chained_handler_and_data() to set
both the handler and its data atomically under the irq_desc->lock.

Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260119063507.940782-1-nichen@iscas.ac.cn