pc-bios: s390x: Use ebcdic2ascii table

Why should we do conversion of a ebcdic value if we have a handy table
where we could look up the ascii value instead?

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200624075226.92728-10-frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>