pc-bios/s390-ccw: fix sclp_get_loadparm_ascii

The existing s390 bios gets the LOADPARM information from the system using
an SCLP call that specifies a buffer length too small to contain all the
output.

The recent fixes in the SCLP code have exposed this bug, since now the
SCLP call will return an error (as per architecture) instead of
writing partially and completing successfully.

The solution is simply to specify the full page length as the SCCB
length instead of a smaller size.

Fixes: 832be0d8 ("s390x: sclp: Report insufficient SCCB length")
Fixes: 9a22473c ("pc-bios/s390-ccw: get LOADPARM stored in SCP Read Info")

Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <1574944437-31182-1-git-send-email-imbrenda@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>