[efi] Open device path protocol only at point of use

[efi] Open device path protocol only at point of use

Some EFI 1.10 systems (observed on an Apple iMac) do not allow us to
open the device path protocol with an attribute of
EFI_OPEN_PROTOCOL_BY_DRIVER and so we cannot maintain a safe,
long-lived pointer to the device path.  Work around this by instead
opening the device path protocol with an attribute of
EFI_OPEN_PROTOCOL_GET_PROTOCOL whenever we need to use it.

Debugged-by: Curtis Larsen <larsen@dixie.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>