[x86 setup] Don't use EDD to get the MBR signature

At least one machine has been identified in the field which advertises
EDD for all drives but locks up if one attempts an extended read from
a non-primary drive.

The MBR is always at CHS 0-0-1, so there is no reason to use an
extended read, other than the possibility that the BIOS cannot handle
it.

Although this might break as many machines as it fixes (a small number
either way), the current state is a regression but the reverse is not.
Therefore revert to the previous state of not using extended read.

Quite probably the Right Thing to do is to read using plain (CHS) read
and extended read on failure, but that change would definitely have to
go through -mm first.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>