drivers: serial: uart_mcux_lpuart: Switch to using DT_INST_IRQN_BY_IDX

After #63289, multi-level interrupts are now encoded using macro
magic. This means that using the generic DT_INST_IRQ_BY_IDX() to
fetch the INTID is no longer an option as the queried INTID will
be the one specified through the node's `interrupts` properties.
To fix this, switch to using DT_INST_IRQN_BY_IDX() which will
return the correctly encoded INTID.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>