gpio: sifive: fix static checker warning

Typcasting "irq_state" leads to the below static checker warning:
The fix is to declare "irq_state" as unsigned long instead of u32.

	drivers/gpio/gpio-sifive.c:97 sifive_gpio_irq_enable()
	warn: passing casted pointer '&chip->irq_state' to
	'assign_bit()' 32 vs 64.

Fixes: 96868dce

 ("gpio/sifive: Add GPIO driver for SiFive SoCs")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Link: https://lore.kernel.org/r/1580189061-14091-1-git-send-email-yash.shah@sifive.com
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>