s390/3270: correct size detection with the read-partition command

The size detection for 3270 terminals with the read-partition command is
broken. The raw3270_reset_device_cb function clears the init_data array,
but if raw3270_writesf_readpart has been called the read-partition command
is queued which needs the init_data array. In this case the size detection
will fail and the invalid command does funny things to the terminal.

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