efi_loader: efi_disk_register() should not fail

Our algorithm for creating USB device paths may lead to duplicate device
paths which result in efi_disk_register() failing. Instead we should just
skip devices that cannot be registered as EFI block devices.

Fix a memory leak in efi_disk_add_dev() caused by the duplicate device
path.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>