netx: Use get_irqnr_preamble to initialize base register

This patch optimizes the irq handling a bit.  Now the base register is
only computed once per irq exception instead of at least twice.
Moreover the mov+add sequence is simplified to an ldr (which might save
some cycles depending on memory timing).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>