efi_loader: refactor efi_uninstall_protocol_interface

For the implementation of UninstallMultipleProtocolInterfaces we
need to call efi_uninstall_protocol_interface. In internal calls
we should not pass through EFI_EXIT.

The patch introduces a wrapper function
efi_uninstall_protocol_interface_ext.

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