gpiolib: cdev: add GPIO_V2_LINE_FLAG_EDGE_BOTH and use it in edge_irq_thread()

Add GPIO_V2_LINE_FLAG_EDGE_BOTH macro and use it in edge_irq_thread() to
improve readability of edge handling cases.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>