s390/3270: use diagnose 0x210 for device sensing under z/VM

There is a debugging leftover from git commit 4d334fd1


"s390/3270: asynchronous size sensing" in raw3270_reset_device_cb.
Under z/VM the diagnose 0x210 can be used to find the correct
size of the 3270 terminal.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>