efi_loader: fix the return codes of UninstallProtocol

Up to now we did not check the return value of DisconnectController.
A previous patch is fixing that taking into account what happened during
the controller disconnect.  But that check takes place before our code
is trying to figure out if the interface exists to begin with.  In case a
driver is not allowed to unbind -- e.g returning EFI_DEVICE_ERROR, we
will end up returning that error instead of EFI_NOT_FOUND.

Add an extra check on the top of the function to make sure the protocol
interface exists before trying to disconnect any drivers

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>