efi_loader: enable file SetInfo()

EFI shell command edit uses the SetInfo() methods to unset the read only
attribute of the file to be edited. So let efi_file_setinfo() return
success in this case.

Return an error if the function is called for to rename or resize a file
as we do not support this yet.

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