ibm_rtl: _RTL_ is not available in UEFI mode

Some of the IBM servers that are supported by ibm_rtl
can run in both Legacy mode (BIOS) and in UEFI mode.
When running in UEFI mode, it is possible that the
EBDA table exists but cannot be mapped and reports
errors.  We need to make sure that by default we don't
try to probe the machines if they are running in UEFI
mode.

Signed-off-by: Vernon Mauery <vernux@us.ibm.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>