irqdomain: Fix irq_create_direct_mapping() to test irq_domain type.

irq_create_direct_mapping can only be used with the NOMAP type.  Make
the function test to ensure it is passed the correct type of
irq_domain.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>