isolinux: Clear upper half of EDX before using

isolinux: Clear upper half of EDX before using

In checkin:

    cb015497a4e4 isolinux: Update LBA in getlinsec loop

... we use EDX as a sector count, but the sector count is actually in
DX, and the upper half of EDX is uninitialized.  If the BIOS enters
with a nonzero value in the upper half of EDX, this breaks horribly.

At least one set of BIOSes has been identified where if the LBA > 64K
then the upper half of EDX will be nonzero.

Reported-by: Carl Duff <cdrw2400@gmail.com>
Reported-by: Philip Müller <philm@manjaro.org>
Tested-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>