gpio: dwapb: Don't use IRQ 0 as valid Linux interrupt

IRQ 0 is not valid in Linux interrupt number space.
Refactor the code with this kept in mind.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20200519131233.59032-3-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>