efi_loader: fix efi_dp_from_name()

Correctly check the return value of efi_dp_from_file().
If we can determine device path for the file, should not depend on the
device path for the device being requested.
Provide a function description for efi_dp_from_name().

Reported-by: Coverity CID 273159, CID 273158
Fixes: 08c51fff ("efi_loader: device_path: check against file path length")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>