s390/con3270: reduce highlight width to 3 bits

With the previous change this reduces the size of struct tty3270_attribute
from four to two bytes. As we have this struct allocated for each character
cell, this saves quite some memory.

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>