[S390] use generic termbits.h header file

A compare shows that arch/s390/include/asm/termbits.h is the same as
include/asm-generic.h. This patch lets arch/s390/include/asm/termbits.h
include the generic header.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>