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>