irqchip/riscv-aplic: Fix an IS_ERR() vs NULL bug in probe()

The devm_platform_ioremap_resource() function doesn't return NULL, it
returns error pointers.  Fix the error handling to match.

Fixes: 2333df5a ("irqchip: Add RISC-V advanced PLIC driver for direct-mode")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/a5a628d6-81d8-4933-81a8-64aad4743ec4@stanley.mountain