gpio: sifive: Fix SiFive gpio probe

Fix the check on the number of IRQs to allow up to the maximum (32)
instead of only the maximum minus one.

Fixes: 96868dce

 ("gpio/sifive: Add GPIO driver for SiFive SoCs")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20201107081420.60325-10-damien.lemoal@wdc.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>