s390/pci: clarify comment in s390_mmio_read/write

The existing comment was talking about reading in the write part
and vice versa. While we are here make it more clear why restricting
the syscalls to MIO capable devices is okay.

Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>