s390/tty3270: rename to con3270

Rename the driver file to con3270 to be in sync with con3215.c.
This removes CONFIG_TN3270_TTY, it is now always build if CONFIG_TN3270
is enabled.

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>