s390/con3270: reduce f_color and b_color attribute size to 4 bit

As we're only supportign 8 colors, we don't need 8 bits. Reduce the
size to 4 bits to save 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>