sh/intc: Use irq_desc_get_xxx() to avoid redundant lookup of irq_desc

sh/intc: Use irq_desc_get_xxx() to avoid redundant lookup of irq_desc

Use irq_desc_get_xxx() to avoid redundant lookup of irq_desc while we
already have a pointer to corresponding irq_desc.

Also replace generic_handle_irq with generic_handle_irq_desc() to avoid
looking up irq_desc again.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20150713151626.792845830@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>