USB: pxa2xx_udc: fix hardcoded irq number

This patch changes last use of hardcoded number of irq to
use platfrom_get_irq.

Signed-off-by: Milan Svoboda <msvoboda@ra.rockwell.com>
Acked-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>