[S390] tape: Fix device online messages

Currently, when a tape device is set online and no cartridge is loaded, we
get the messages "The tape cartridge has been successfully unloaded" and
"Determining the size of the recorded area". These messages are not correct.
To fix this, we now print the "cartridge loaded/unloaded" messages only,
when the load/unload event really occurs. In addition to that, the message
"Determining the size of the recorded area" is only printed, if a cartridge
is loaded.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>