efi_loader: don't use HandleProtocol

HandleProtocol() is deprecrated. According to the UEFI specification it
should be implemented as a call to  OpenProtocolInterface() with a hard
coded agent handle. This implies that we would have to call
CloseProtocolInterfaces() after usage with the same handle.

Getting rid of an EFI_CALL() is also appreciated.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>