s390/con3215: Simplify console write operation

The functions con3215_write() and tty3215_write() have nearly
identical function bodies and a slightly different function prototype.
Create function handle_write() to handle the common function
body and maintain the function prototypes.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>