irqchip/qcom-mpm: Fix IS_ERR() vs NULL check in qcom_mpm_init()

devm_ioremap() doesn't return error pointers, it returns NULL on error.
Update the check accordingly.

Fixes: 221b110d ("irqchip/qcom-mpm: Support passing a slice of SRAM as reg space")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/22e1f4de-edce-4791-bd2d-2b2e98529492@moroto.mountain