of/irq: little endian fixes

Fix some endian issues in the irq mapping OF code.

Signed-off-by: Rob Herring <r.herring@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
CC: Michal Simek <monstr@monstr.eu>
CC: Wolfram Sang <w.sang@pengutronix.de>
CC: Stephen Rothwell <sfr@canb.auug.org.au>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>