[efi] Use the SNP protocol instance to match the SNP chainloading device

Some systems will install a child of the SNP device and use this as
our loaded image's device handle, duplicating the installation of the
underlying SNP protocol onto the child device handle.  On such
systems, we want to end up driving the parent device (and
disconnecting any other drivers, such as MNP, which may be attached to
the parent device).

Fix by recording the SNP protocol instance at initialisation time, and
using this to match against device handles (rather than simply
comparing the handles themselves).

Reported-by: Jarrod Johnson <jarrod.b.johnson@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>