s390/pci: use proper endianness annotations

Add proper annotation to the bar definition and use casts within the
bus accessors. Also change the sequence in the accessors to do the
shifts in the native byte order. No functional change.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>