drivers: interrupt_controller: intc_plic: rewrite `get_plic_dev_from_irq`

There's a ternary operator that depends on configuration-defined macro:
`CONFIG_DYNAMIC_INTERRUPTS` is not enabled by default
for any of the platforms that use PLIC,
it is possbile to set it to `=y` though.

This triggered the Coverity check to report it as dead code.

Fixes #65576.

Signed-off-by: Wojciech Sipak <wsipak@antmicro.com>