tty: serial: lpuart: avoid report NULL interrupt

The current driver register irq in .startup() and free the irq in
.shutdown(), then user will see the NULL interrupt output from
'cat /proc/interrupts' after the uart port test completed:
...
 41:        515          0          0          0     GICv3 257 Level     fsl-lpuart
 42:          2          0          0          0     GICv3 258 Level
...

It is better to register all the irqs during probe function via devm_request_irq()
to avoid to call free_irq().

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>