s390/fs3270: fix screen reset on activate

fs3270 uses EWRITEA to clear the screen when a user opens /dev/3270/tub.
However it misses the attribute byte after the EWRITEA, so (at least)
x3270 complains about 'Record too short, missing write flags'.
Add the missing flag byte to fix this.

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>