iio: generic_buffer: be helpful about enabling channels

Currently if generic_buffer is invoked without first enabling any
channels in scan_elements/*_en, it will fail unable to enable the
buffer because bytes_per_datum inside the kernel will be zero if
no channels are available.

It is implied that the user of the program should enable channels
manually or with a script before executing generic_buffer.

Be more helpful by stopping execution if no enabled channels can
be found, and print a helptext that will tell you what is wrong
and what needs to be done.

Signed-off-by: Linus Walleij <[e-mail]>
Acked-by: Hartmut Knaack <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

tools/iio/generic_buffer.c

···
 			"diag %s\n", dev_dir_name);
 		goto error_free_triggername;
 	}
+	if (!num_channels) {
+		fprintf(stderr,
+			"No channels are enabled, we have nothing to scan.\n");
+		fprintf(stderr, "Enable channels manually in "
+			FORMAT_SCAN_ELEMENTS_DIR
+			"/*_en and try again.\n", dev_dir_name);
+		ret = -ENOENT;
+		goto error_free_triggername;
+	}
 
 	/*
 	 * Construct the directory name for the associated buffer.