of: irq: fix of_irq_get[_byname]() kernel-doc

The kernel-doc for the of_irq_get[_byname]()  is clearly inadequate in
describing the return values -- of_irq_get_byname() is documented better
than of_irq_get() but it  still doesn't mention that 0 is returned iff
irq_create_of_mapping() fails (it doesn't return an error code in this
case). Document all possible return value variants, making the writing
of the word "IRQ" consistent, while at it...

Fixes: 9ec36caf ("of/irq: do irq resolution in platform_get_irq")
Fixes: ad69674e ("of/irq: do irq resolution in platform_get_irq_byname()")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
CC: stable@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>