efi_loader: efi_handle_protocol set attributes

UEFI spec 2.7 indicates that HandleProtocol can be implemented
by calling OpenProtocol with
attributes = EFI_OPEN_PROTOCOL_BY_HANDLE_PROTOCOL.

Currently we pass attributes = 0 to efi_open_protocol. 0 is not a
valid value when calling OpenProtocol. This does not cause any errors
yet because our implementation of OpenProtocol is incomplete.

We should pass the correct value to enable a fully compliant
implementation of OpenProtocol in the future.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>