s390/3270: unify con3270 + tty3270

There were two flavours of 3270 drivers: con3270, which provides a
console driver, and tty3270 providing a tty driver. A user could switch
with PF3 between the two views. While this sounds nice, it's a bit
annoying: If the user enters a command which triggers a kernel message,
he always has to switch back and forth to see the message. Unify both
views to have kernel messages and tty in one screen, like it is on almost
all other platforms. This also has the nice side effect of removing a lot
of duplicate code.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>