s390/stp: use u32 instead of unsigned int

In hardware-dependent headers using u32 is easier
to read and less error-prone.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>