eltorito: Scan upwards instead of downwards

eltorito: Scan upwards instead of downwards

A developer for GRUB4DOS called tinybit has reported that some
BIOSes crash when the DOS ElTorito.Sys driver scans drive number
0xFF.  According to the El Torito specification, drive 0x7F
means "terminate all," so a plausible explanation suggested
by H. Peter Anvin is that the drive 0xFF is a shadow of drive
0x7F, thus contributing to a failure on these BIOSes.

Now we scan from 0x80 through 0xFF, instead.

[ hpa: dropped checkin of eltorito.sys ]

Signed-off-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>