s390x: uv: Fix UVC cmd prepare reset name

The specification and the kernel use UVC_CMD_PREPARE_RESET so let's
fix our naming up.

The call bit is named correctly but is not the same as the name we use
on KVM. So let's clear that up too.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>