efi_loader: LocateDevicePath() incorrect parameter check

A parameter check in LocateDevicePath() does not match the requirements of
the UEFI spec.

If device is NULL, only return EFI_INVALID_PARAMETER if a matching handle
is found.

Cf. UEFI SCT II specification (2017)3.3.7 LocateDevicePath(), 5.1.3.7.3

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