irqchip: omap-intc: Improve IRQ handler

As it turns out the current IRQ number will *always* be available from
SIR register which renders the reads of PENDING registers as plain
unnecessary overhead.

In order to catch any situation where SIR reads as zero, we're adding
a WARN() to turn it into a very verbose error and users actually
report it.

With this patch average running time of omap_intc_handle_irq() reduced
from about 28.5us to 19.8us as measured by the kernel function
profiler.

Tested with BeagleBoneBlack Rev A5C.

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Cc: Linux ARM Kernel Mailing List <linux-arm-kernel@lists.infradead.org>
Link: http://lkml.kernel.org/r/20150720204910.GH5394@saruman.tx.rr.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>