s390/pci: avoid using strncmp with hardcoded length

Command line option values passed to __setup callbacks are always
null-terminated and "s390_iommu=" may only accept "strict" as value.
So replace strncmp with strcmp.

While at it also make s390_iommu_setup return 1, which means this
command line option is handled by this callback.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/pci/pci_dma.c

@@ -674,9 +674,9 @@ EXPORT_SYMBOL_GPL(s390_pci_dma_ops);
 
 static int __init s390_iommu_setup(char *str)
 {
-	if (!strncmp(str, "strict", 6))
+	if (!strcmp(str, "strict"))
 		s390_iommu_strict = 1;
-	return 0;
+	return 1;
 }
 
 __setup("s390_iommu=", s390_iommu_setup);