[int13] Do not report INT 13 extension support for emulated floppies

[int13] Do not report INT 13 extension support for emulated floppies

The INT 13 extensions provide a mechanism for accessing disks using
linear (LBA) rather than C/H/S addressing.  SAN protocols such as
iSCSI invariably support only linear addresses and so iPXE currently
provides LBA access to all SAN disks (with autodetection and emulation
of an appropriate geometry for C/H/S accesses).

Most BIOSes will not report support for INT 13 extensions for floppy
disk drives, and some operating systems may be confused by a floppy
drive that claims such support.

Minimise surprise by reporting the existence of support for INT 13
extensions only for non-floppy drive numbers.  Continue to provide
support for all drive numbers, to avoid breaking operating systems
that may unconditionally use the INT 13 extensions without first
checking for support.

Reported-by: Valdo Toost <vtoost@hot.ee>
Signed-off-by: Michael Brown <mcb30@ipxe.org>