gpio: dwapb: Switch to more usual pattern of RMW in dwapb_gpio_set_debounce()

gpio: dwapb: Switch to more usual pattern of RMW in dwapb_gpio_set_debounce()

More usual pattern is to prepare value and then write it in a single place.
Switch code in dwapb_gpio_set_debounce() to it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20200415141534.31240-10-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>