s390/uv: Use a constant for more-data rc

s390/uv: Use a constant for more-data rc

Add a define for the UVC rc 0x0100 that indicates that a UV-call was
successful but may serve more data if called with a larger buffer
again.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20241024062638.1465970-2-seiden@linux.ibm.com
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>