gpio: 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.

Preparatory patch for the removal of the 'irq' argument from irq flow
handlers.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>