power: reset: xgene-reboot: Fix a NULL vs IS_ERR() test

The devm_platform_ioremap_resource() function returns error points.  It
never returns NULL.  Update the check accordingly.

Fixes: 7ddfd33c ("power: reset: xgene-reboot: Use devm_platform_ioremap_resource() helper")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/fe1b90d0-7234-4e03-accc-69a119f6a7eb@moroto.mountain


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>