[S390] tape: remove f_pos changes from read / write function.

The change to f_pos in the read / write method has no effect as the
value is controlled by the VFS layer, e.g. for vfs_read:

	loff_t pos = file_pos_read(file);
	ret = vfs_read(file, buf, count, &pos);
	file_pos_write(file, pos);

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>