efi_loader: error code in UninstallProtocolInterface()

According to the UEFI specification UninstallProtocolInteface() has to
return EFI_NOT_FOUND if the interface is not found.

Correct the return value.

Cf. UEFI SCT II spec (2017), 3.3.2 UninstallProtocolInterface(), 5.1.3.2.4

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>