misc/pvpanic: fix a NULL vs IS_ERR() check

The devm_ioremap_resource() function doesn't return NULL, it returns
error pointers.

Fixes: 46f934c9 ("misc/pvpanic: add support to get pvpanic device info FDT")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>