[bios] Fix screen clearing on buggy BIOSes

The implementation of INT 10,06 on some BIOSes (observed with both
Hyper-V and a Dell OptiPlex 7010) seems to treat %dx=0xffff as a
special value meaning "do absolutely nothing".  Fix by using
%dx=0xfefe, which should still be sufficient to cover any realistic
screen size.

Reported-by: John Clark <skyman@iastate.edu>
Tested-by: John Clark <skyman@iastate.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>