s390/con3270: fix camelcase in enum members

fix the following and similar checkpatch warnings:

CHECK: Avoid CamelCase: <ESnormal>
+       enum { ESnormal, ESesc, ESsquare, ESparen, ESgetpars };

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>