staging: greybus: loopback_test: use octal permissions instead of symbolic

checkpatch.pl warning:
Symbolic permissions are not preferred. Consider using octal permissions.

Signed-off-by: Igor Pylypiv <igor.pylypiv@gmail.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>