pinctrl: mcp23s08: Fix missing unlock on error in mcp23s08_irq()

Add the missing unlock before return from function mcp23s08_irq()
in the error handling case.

v1-->v2:
   remove the "return IRQ_HANDLED" line

Fixes: 897120d4

 ("pinctrl: mcp23s08: fix race condition in irq handler")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1623134048-56051-1-git-send-email-zou_wei@huawei.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>